diff --git a/README.md b/README.md
index 80f2e31..52e690e 100644
--- a/README.md
+++ b/README.md
@@ -10,4 +10,5 @@ Implementation status:
 
 [Draft Spec](https://w3c.github.io/IntersectionObserver/)
 
-[Polyfill](./polyfill/)
+Given that multiple native implementations of IntersectionObserver exist today, the polyfill that
+was formerly maintained in this repo is no longer supported and has been archived at https://github.com/GoogleChromeLabs/intersection-observer.
diff --git a/polyfill/.eslintrc b/polyfill/.eslintrc
deleted file mode 100644
index 3cac2ba..0000000
--- a/polyfill/.eslintrc
+++ /dev/null
@@ -1,45 +0,0 @@
-{
-  "env": {
-    "browser": true,
-    "node": true,
-    "mocha": true
-  },
-  "globals": {
-    "expect": false,
-    "sinon": false
-  },
-  "extends": [
-    "eslint:recommended"
-  ],
-  "rules": {
-    // possible errors
-    "no-cond-assign": 0,
-    "no-empty": 0,
-    "valid-jsdoc": [2, {
-      "prefer": {"return": "return"},
-      "requireReturn": false
-    }],
-    // best practices
-    "no-extra-bind": 2,
-    // style
-    "comma-spacing": [2, {"before": false, "after": true}],
-    "eol-last": [0],
-    "key-spacing": [2, {"beforeColon": false, "afterColon": true}],
-    "linebreak-style": [2, "unix"],
-    "max-len": [2, 80, {"ignoreUrls": true}],
-    "no-multiple-empty-lines": [2, {"max": 2}],
-    "no-unused-vars": ["error", {"vars": "all"}],
-    "no-trailing-spaces": 2,
-    "object-curly-spacing": [2, "never"],
-    "quotes": [2, "single"],
-    "require-jsdoc": [2, {
-      "require": {
-        "FunctionDeclaration": true,
-        "MethodDefinition": true,
-        "ClassDeclaration": true
-      }
-    }],
-    "semi": [2, "always"],
-    "semi-spacing": [2, {"before": false, "after": true}],
-  }
-}
\ No newline at end of file
diff --git a/polyfill/.gitignore b/polyfill/.gitignore
deleted file mode 100644
index 24000a8..0000000
--- a/polyfill/.gitignore
+++ /dev/null
@@ -1,3 +0,0 @@
-# Sauce Labs testing files
-easy-sauce-config.json
-sauce.sh
diff --git a/polyfill/README.md b/polyfill/README.md
deleted file mode 100644
index 6d9bcb2..0000000
--- a/polyfill/README.md
+++ /dev/null
@@ -1,213 +0,0 @@
-# `IntersectionObserver` polyfill
-
-This library polyfills the native [`IntersectionObserver`](http://w3c.github.io/IntersectionObserver/) API in unsupporting browsers. See the [API documentation](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) for usage information.
-
-- [Installation](#installation)
-- [Configuring the polyfill](#configuring-the-polyfill)
-- [Limitations](#limitations)
-- [Browser support](#browser-support)
-- [Running the tests](#running-the-tests)
-
-## Installation
-
-You can install the polyfill via npm or by downloading a [zip](https://github.com/w3c/IntersectionObserver/archive/gh-pages.zip) of this repository:
-
-```sh
-npm install intersection-observer
-```
-
-### Adding the polyfill to your site
-
-The examples below show various ways to add the `IntersectionObserver` polyfill to your site. Be sure to include the polyfill prior to referencing it anywhere in your JavaScript code.
-
-**Using `<script>` tags in the HTML:**
-
-```html
-<!-- Load the polyfill first. -->
-<script src="path/to/intersection-observer.js"></script>
-
-<!-- Load all other JavaScript. -->
-<script src="app.js"></script>
-```
-
-**Using a module loader (e.g. Browserify or Webpack):**
-
-```js
-// Require the polyfill before requiring any other modules.
-require('intersection-observer');
-
-require('./foo.js');
-require('./bar.js');
-```
-
-## Configuring the polyfill
-
-It's impossible to handle all possible ways a target element could intersect with a root element without resorting to constantly polling the document for intersection changes.
-
-To avoid this extra work and performance penalty, the default configuration of the polyfill optimizes for the most common `IntersectionObserver` use cases, which primarily include target elements intersecting with a root element due to:
-
-- User scrolling.
-- Resizing the window.
-- Changes to the DOM.
-
-All of the above can be handled without polling the DOM.
-
-There are, however, additional use cases that the default configuration will not detect. These include target elements intersecting with a root element due to:
-
-- CSS changes on `:hover`, `:active`, or `:focus` states.
-- CSS changes due to transitions or animations with a long initial delay.
-- Resizable `<textarea>` elements that cause other elements to move around.
-- Scrolling of non-document elements in browsers that don't support the event capture phase.
-
-If you need to handle any of these use-cases, you can configure the polyfill to poll the document by setting the `POLL_INTERVAL` property. This can be set either globally or on a per-instance basis.
-
-**Enabling polling for all instances:**
-
-To enable polling for all instances, set a value for `POLL_INTERVAL` on the `IntersectionObserver` prototype:
-
-
-```js
-IntersectionObserver.prototype.POLL_INTERVAL = 100; // Time in milliseconds.
-```
-
-**Enabling polling for individual instances:**
-
-To enable polling on only specific instances, set a `POLL_INTERVAL` value on the instance itself:
-
-```js
-var io = new IntersectionObserver(callback);
-io.POLL_INTERVAL = 100; // Time in milliseconds.
-io.observe(someTargetElement);
-```
-
-**Note:** the `POLL_INTERVAL` property must be set prior to calling the `.observe` method, or the default configuration will be used.
-
-**Ignoring DOM changes**
-
-You can also choose to not check for intersections when the DOM changes by setting an observer's `USE_MUTATION_OBSERVER` property to `false` (either globally on the prototype or per-instance)
-
-```js
-IntersectionObserver.prototype.USE_MUTATION_OBSERVER = false; // Globally
-
-// for an instance
-var io = new IntersectionObserver(callback);
-io.USE_MUTATION_OBSERVER = false;
-```
-
-This is recommended in cases where the DOM will update frequently but you know those updates will have no effect on the position or your target elements.
-
-
-## iframe support
-
-### Same-origin iframes
-
-Same-origin iframes are supported by the polyfill out of the box.
-
-
-### Cross-origin iframes
-
-Additional code and configuration are required to support cross-origin iframes,
-both on the iframe and host sides.
-
-The setup is as following:
-
-1. The host and iframe will establish a messaging channel.
-2. The host will setup its own IntersectionObserver instance for the
-cross-origin iframe element. It can either use the this polyfill or any other
-approach. For each IntersectionObserverEntry for the iframe it will forward
-intersection data to the iframe via messaging.
-3. The iframe will load the polyfill and configure it by calling the
-`_setupCrossOriginUpdater()` method. It will call the provided callback
-whenever it receives the intersection data from the the parent via messaging.
-
-A hypothetical host code:
-
-```javascript
-function forwardIntersectionToIframe(iframe) {
-  createMessagingChannel(iframe, function(port) {
-    var io = new IntersectionObserver(function() {
-      port.postMessage({
-        boundingClientRect: serialize(boundingClientRect),
-        intersectionRect: serialize(intersectionRect)
-      });
-    }, {threshold: [0, 0.1, ..., 1]});
-    io.observe(iframe);
-  });
-}
-```
-
-Notice that the host should provide a `threshold` argument for the desired
-level of precision. Otherwise, the iframe side may not update as frequently as
-desired.
-
-A hypothetical iframe code:
-
-```javascript
-createMessagingChannel(parent, function(port) {
-  if (IntersectionObserver._setupCrossOriginUpdater) {
-    var crossOriginUpdater = IntersectionObserver._setupCrossOriginUpdater();
-    port.onmessage = function(event) {
-      crossOriginUpdater(
-        deserialize(event.data.boundingClientRect),
-        deserialize(event.data.intersectionRect)
-      );
-    };
-  }
-});
-```
-
-
-## Limitations
-
-This polyfill does not support the [proposed v2 additions](https://github.com/szager-chromium/IntersectionObserver/blob/v2/explainer.md), as these features are not currently possible to do with JavaScript and existing web APIs.
-
-## Browser support
-
-The polyfill has been tested and known to work in the latest version of all browsers.
-
-Legacy support is also possible in very old browsers by including a shim for ES5 as well as the `window.getComputedStyle` method. The easiest way to load the IntersectionObserver polyfill and have it work in the widest range of browsers is via [polyfill.io](https://cdn.polyfill.io/v3/), which will automatically include dependencies where necessary:
-
-```html
-<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>
-```
-
-With these polyfills, `IntersectionObserver` has been tested and known to work in the following browsers:
-
-<table>
-  <tr>
-    <td align="center">
-      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/chrome/chrome_48x48.png" alt="Chrome"><br>
-      ✔
-    </td>
-    <td align="center">
-      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/firefox/firefox_48x48.png" alt="Firefox"><br>
-      ✔
-    </td>
-    <td align="center">
-      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/safari/safari_48x48.png" alt="Safari"><br>
-      6+
-    </td>
-    <td align="center">
-      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/edge/edge_48x48.png" alt="Edge"><br>
-      ✔
-    </td>
-    <td align="center">
-      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/archive/internet-explorer_7-8/internet-explorer_7-8_48x48.png" alt="Internet Explorer"><br>
-      7+
-    </td>
-    <td align="center">
-      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/opera/opera_48x48.png" alt="Opera"><br>
-      ✔
-    </td>
-    <td align="center">
-      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/android/android_48x48.png" alt="Android"><br>
-      4.4+
-    </td>
-  </tr>
-</table>
-
-## Running the tests
-
-To run the test suite for the `IntersectionObserver` polyfill, open the [`intersection-observer-test.html`](./intersection-observer-test.html) page in the browser of your choice.
-
-If you run the tests in a browser that supports `IntersectionObserver` natively, the tests will be run against the native implementation. If it doesn't, the tests will be run against the polyfill.
diff --git a/polyfill/intersection-observer-test.html b/polyfill/intersection-observer-test.html
deleted file mode 100644
index 3a2b7ca..0000000
--- a/polyfill/intersection-observer-test.html
+++ /dev/null
@@ -1,73 +0,0 @@
-<!--
-Copyright 2016 Google Inc. All Rights Reserved.
-
-Licensed under the W3C SOFTWARE AND DOCUMENT NOTICE AND LICENSE.
-
- https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document
--->
-<!DOCTYPE html>
-<html lang="en">
-<head>
-  <meta charset="utf-8">
-  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
-  <title>IntersectionObserver Tests</title>
-
-  <!-- Dependencies -->
-  <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/mocha/2.5.3/mocha.min.css">
-  <script src="https://cdnjs.cloudflare.com/ajax/libs/mocha/2.5.3/mocha.min.js"></script>
-  <script src="https://cdnjs.cloudflare.com/ajax/libs/expect.js/0.2.0/expect.min.js"></script>
-  <script src="https://cdnjs.cloudflare.com/ajax/libs/sinon.js/3.2.1/sinon.min.js"></script>
-
-  <!-- Polyfills for IE7-8 -->
-  <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=es5,getComputedStyle"></script>
-</head>
-<body>
-  <div id="mocha"></div>
-
-  <!-- Setup -->
-  <script>mocha.setup({ui:'bdd'});</script>
-
-  <!-- Uncomment this script to force the polyfill installation
-  <script>
-    delete window.IntersectionObserver;
-    delete window.IntersectionObserverEntry;
-  </script>
-  -->
-
-  <!-- Script under test -->
-  <script src="intersection-observer.js"></script>
-
-  <!-- Tests -->
-  <script src="intersection-observer-test.js"></script>
-
-  <!-- Reporting -->
-  <script>
-  var runner = mocha.run();
-  var failedTests = [];
-
-  runner.on('end', function(){
-    window.mochaResults = runner.stats;
-    window.mochaResults.reports = failedTests;
-  });
-
-  runner.on('fail', function(test, err){
-    var flattenTitles = function(test){
-      var titles = [];
-      while (test.parent.title){
-        titles.push(test.parent.title);
-        test = test.parent;
-      }
-      return titles.reverse();
-    };
-
-    failedTests.push({
-      name: test.title,
-      result: false,
-      message: err.message,
-      stack: err.stack,
-      titles: flattenTitles(test)
-    });
-  });
-  </script>
-</body>
-</html>
diff --git a/polyfill/intersection-observer-test.js b/polyfill/intersection-observer-test.js
deleted file mode 100644
index 79666c1..0000000
--- a/polyfill/intersection-observer-test.js
+++ /dev/null
@@ -1,3329 +0,0 @@
-/**
- * Copyright 2016 Google Inc. All Rights Reserved.
- *
- * Licensed under the W3C SOFTWARE AND DOCUMENT NOTICE AND LICENSE.
- *
- *  https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document
- *
- */
-
-
-// Sets the timeout to three times the poll interval to ensure all updates
-// happen (especially in slower browsers). Native implementations get the
-// standard 100ms timeout defined in the spec.
-var ASYNC_TIMEOUT = IntersectionObserver.prototype.THROTTLE_TIMEOUT * 3 || 100;
-
-
-var io;
-var noop = function() {};
-
-
-// References to DOM elements, which are accessible to any test
-// and reset prior to each test so state isn't shared.
-var rootEl;
-var grandParentEl;
-var parentEl;
-var targetEl1;
-var targetEl2;
-var targetEl3;
-var targetEl4;
-
-
-describe('IntersectionObserver', function() {
-
-  before(function() {
-    // If the browser running the tests doesn't support MutationObserver,
-    // fall back to polling.
-    if (!('MutationObserver' in window)) {
-      IntersectionObserver.prototype.POLL_INTERVAL =
-          IntersectionObserver.prototype.THROTTLE_TIMEOUT || 100;
-    }
-  });
-
-
-  beforeEach(function() {
-    addStyles();
-    addFixtures();
-  });
-
-
-  afterEach(function() {
-    if (io && 'disconnect' in io) io.disconnect();
-    io = null;
-
-    removeStyles();
-    removeFixtures();
-  });
-
-
-  describe('constructor', function() {
-
-    it('throws when callback is not a function', function() {
-      expect(function() {
-        io = new IntersectionObserver(null);
-      }).to.throwException();
-    });
-
-
-    it('instantiates root correctly', function() {
-      io = new IntersectionObserver(noop);
-      expect(io.root).to.be(null);
-
-      io = new IntersectionObserver(noop, {root: document});
-      expect(io.root).to.be(document);
-
-      io = new IntersectionObserver(noop, {root: rootEl});
-      expect(io.root).to.be(rootEl);
-    });
-
-
-    it('throws when root is not a Document or Element', function() {
-      expect(function() {
-        io = new IntersectionObserver(noop, {root: 'foo'});
-      }).to.throwException();
-    });
-
-
-    it('instantiates rootMargin correctly', function() {
-      io = new IntersectionObserver(noop, {rootMargin: '10px'});
-      expect(io.rootMargin).to.be('10px 10px 10px 10px');
-
-      io = new IntersectionObserver(noop, {rootMargin: '10px -5%'});
-      expect(io.rootMargin).to.be('10px -5% 10px -5%');
-
-      io = new IntersectionObserver(noop, {rootMargin: '10px 20% 0px'});
-      expect(io.rootMargin).to.be('10px 20% 0px 20%');
-
-      io = new IntersectionObserver(noop, {rootMargin: '0px 0px -5% 5px'});
-      expect(io.rootMargin).to.be('0px 0px -5% 5px');
-
-      // TODO(philipwalton): the polyfill supports fractional pixel and
-      // percentage values, but the native Chrome implementation does not,
-      // at least not in what it reports `rootMargin` to be.
-      if (!supportsNativeIntersectionObserver()) {
-        io = new IntersectionObserver(noop, {rootMargin: '-2.5% -8.5px'});
-        expect(io.rootMargin).to.be('-2.5% -8.5px -2.5% -8.5px');
-      }
-    });
-
-
-    // TODO(philipwalton): this doesn't throw in FF, consider readding once
-    // expected behavior is clarified.
-    // it('throws when rootMargin is not in pixels or pecernt', function() {
-    //   expect(function() {
-    //     io = new IntersectionObserver(noop, {rootMargin: '0'});
-    //   }).to.throwException();
-    // });
-
-
-    // Chrome's implementation in version 51 doesn't include the thresholds
-    // property, but versions 52+ do.
-    if ('thresholds' in IntersectionObserver.prototype) {
-      it('instantiates thresholds correctly', function() {
-        io = new IntersectionObserver(noop);
-        expect(io.thresholds).to.eql([0]);
-
-        io = new IntersectionObserver(noop, {threshold: 0.5});
-        expect(io.thresholds).to.eql([0.5]);
-
-        io = new IntersectionObserver(noop, {threshold: [0.25, 0.5, 0.75]});
-        expect(io.thresholds).to.eql([0.25, 0.5, 0.75]);
-
-        io = new IntersectionObserver(noop, {threshold: [1, .5, 0]});
-        expect(io.thresholds).to.eql([0, .5, 1]);
-      });
-    }
-
-
-    it('throws when a threshold is not a number', function() {
-      expect(function() {
-        io = new IntersectionObserver(noop, {threshold: ['foo']});
-      }).to.throwException();
-    });
-
-
-    it('throws when a threshold value is not between 0 and 1', function() {
-      expect(function() {
-        io = new IntersectionObserver(noop, {threshold: [0, -1]});
-      }).to.throwException();
-    });
-
-  });
-
-
-  describe('observe', function() {
-
-    it('throws when target is not an Element', function() {
-      expect(function() {
-        io = new IntersectionObserver(noop);
-        io.observe(null);
-      }).to.throwException();
-    });
-
-    it('fills in x and y in the resulting rects', function(done) {
-      io = new IntersectionObserver(function(records) {
-        expect(records.length).to.be(1);
-        var entry = records[0];
-        expect(entry.rootBounds.x).to.be(entry.rootBounds.left);
-        expect(entry.rootBounds.y).to.be(entry.rootBounds.top);
-        expect(entry.boundingClientRect.x).to.be(entry.boundingClientRect.left);
-        expect(entry.boundingClientRect.y).to.be(entry.boundingClientRect.top);
-        expect(entry.intersectionRect.x).to.be(entry.intersectionRect.left);
-        expect(entry.intersectionRect.y).to.be(entry.intersectionRect.top);
-        done();
-      }, {root: rootEl});
-      targetEl2.style.top = '-40px';
-      io.observe(targetEl1);
-    });
-
-    it('triggers for all targets when observing begins', function(done) {
-      io = new IntersectionObserver(function(records) {
-        expect(records.length).to.be(2);
-        expect(records[0].intersectionRatio).to.be(1);
-        expect(records[1].intersectionRatio).to.be(0);
-        done();
-      }, {root: rootEl});
-
-      targetEl2.style.top = '-40px';
-      io.observe(targetEl1);
-      io.observe(targetEl2);
-    });
-
-    it('triggers for existing targets when observing begins after monitoring has begun', function(done) {
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl});
-
-      io.observe(targetEl1);
-      setTimeout(function() {
-        io.observe(targetEl2);
-        setTimeout(function() {
-          expect(spy.callCount).to.be(2);
-          done();
-        }, ASYNC_TIMEOUT);
-      }, ASYNC_TIMEOUT);
-    });
-
-
-    it('triggers with the correct arguments', function(done) {
-      io = new IntersectionObserver(function(records, observer) {
-        expect(records.length).to.be(2);
-        expect(records[0] instanceof IntersectionObserverEntry).to.be.ok();
-        expect(records[1] instanceof IntersectionObserverEntry).to.be.ok();
-        expect(observer).to.be(io);
-        expect(this).to.be(io);
-        done();
-      }, {root: rootEl});
-
-      targetEl2.style.top = '-40px';
-      io.observe(targetEl1);
-      io.observe(targetEl2);
-    });
-
-
-    it('handles container elements with non-visible overflow',
-        function(done) {
-
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl});
-
-      runSequence([
-        function(done) {
-          io.observe(targetEl1);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.left = '-40px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(0);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          parentEl.style.overflow = 'visible';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(3);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-    });
-
-
-    it('observes one target at a single threshold correctly', function(done) {
-
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl, threshold: 0.5});
-
-      runSequence([
-        function(done) {
-          targetEl1.style.left = '-5px';
-          io.observe(targetEl1);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be.greaterThan(0.5);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.left = '-15px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be.lessThan(0.5);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.left = '-25px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.left = '-10px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(3);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(0.5);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-
-    });
-
-
-    it('observes multiple targets at multiple thresholds correctly',
-        function(done) {
-
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {
-        root: rootEl,
-        threshold: [1, 0.5, 0]
-      });
-
-      runSequence([
-        function(done) {
-          targetEl1.style.top = '0px';
-          targetEl1.style.left = '-15px';
-          targetEl2.style.top = '-5px';
-          targetEl2.style.left = '0px';
-          targetEl3.style.top = '0px';
-          targetEl3.style.left = '205px';
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          io.observe(targetEl3);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(3);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(0.25);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(0.75);
-            expect(records[2].target).to.be(targetEl3);
-            expect(records[2].intersectionRatio).to.be(0);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.top = '0px';
-          targetEl1.style.left = '-5px';
-          targetEl2.style.top = '-15px';
-          targetEl2.style.left = '0px';
-          targetEl3.style.top = '0px';
-          targetEl3.style.left = '195px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(3);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(0.75);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(0.25);
-            expect(records[2].target).to.be(targetEl3);
-            expect(records[2].intersectionRatio).to.be(0.25);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.top = '0px';
-          targetEl1.style.left = '5px';
-          targetEl2.style.top = '-25px';
-          targetEl2.style.left = '0px';
-          targetEl3.style.top = '0px';
-          targetEl3.style.left = '185px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(3);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(3);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(1);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(0);
-            expect(records[2].target).to.be(targetEl3);
-            expect(records[2].intersectionRatio).to.be(0.75);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.top = '0px';
-          targetEl1.style.left = '15px';
-          targetEl2.style.top = '-35px';
-          targetEl2.style.left = '0px';
-          targetEl3.style.top = '0px';
-          targetEl3.style.left = '175px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(4);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].target).to.be(targetEl3);
-            expect(records[0].intersectionRatio).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-    });
-
-
-    it('handles rootMargin properly', function(done) {
-
-      parentEl.style.overflow = 'visible';
-      targetEl1.style.top = '0px';
-      targetEl1.style.left = '-20px';
-      targetEl2.style.top = '-20px';
-      targetEl2.style.left = '0px';
-      targetEl3.style.top = '0px';
-      targetEl3.style.left = '200px';
-      targetEl4.style.top = '180px';
-      targetEl4.style.left = '180px';
-
-      runSequence([
-        function(done) {
-          io = new IntersectionObserver(function(records) {
-            records = sortRecords(records);
-            expect(records.length).to.be(4);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(1);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(.5);
-            expect(records[2].target).to.be(targetEl3);
-            expect(records[2].intersectionRatio).to.be(.5);
-            expect(records[3].target).to.be(targetEl4);
-            expect(records[3].intersectionRatio).to.be(1);
-            io.disconnect();
-            done();
-          }, {root: rootEl, rootMargin: '10px'});
-
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          io.observe(targetEl3);
-          io.observe(targetEl4);
-        },
-        function(done) {
-          io = new IntersectionObserver(function(records) {
-            records = sortRecords(records);
-            expect(records.length).to.be(4);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(0.5);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(0);
-            expect(records[2].target).to.be(targetEl3);
-            expect(records[2].intersectionRatio).to.be(0.5);
-            expect(records[3].target).to.be(targetEl4);
-            expect(records[3].intersectionRatio).to.be(0.5);
-
-            io.disconnect();
-            done();
-          }, {root: rootEl, rootMargin: '-10px 10%'});
-
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          io.observe(targetEl3);
-          io.observe(targetEl4);
-        },
-        function(done) {
-          io = new IntersectionObserver(function(records) {
-            records = sortRecords(records);
-            expect(records.length).to.be(4);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(0.5);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(0);
-            expect(records[2].target).to.be(targetEl3);
-            expect(records[2].intersectionRatio).to.be(0);
-            expect(records[3].target).to.be(targetEl4);
-            expect(records[3].intersectionRatio).to.be(0.5);
-            io.disconnect();
-            done();
-          }, {root: rootEl, rootMargin: '-5% -2.5% 0px'});
-
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          io.observe(targetEl3);
-          io.observe(targetEl4);
-        },
-        function(done) {
-          io = new IntersectionObserver(function(records) {
-            records = sortRecords(records);
-            expect(records.length).to.be(4);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(0.5);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(0.5);
-            expect(records[2].target).to.be(targetEl3);
-            expect(records[2].intersectionRatio).to.be(0);
-            expect(records[3].target).to.be(targetEl4);
-            expect(records[3].intersectionRatio).to.be(0.25);
-            io.disconnect();
-            done();
-          }, {root: rootEl, rootMargin: '5% -2.5% -10px -190px'});
-
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          io.observe(targetEl3);
-          io.observe(targetEl4);
-        }
-      ], done);
-    });
-
-
-    it('handles targets on the boundary of root', function(done) {
-
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl});
-
-      runSequence([
-        function(done) {
-          targetEl1.style.top = '0px';
-          targetEl1.style.left = '-21px';
-          targetEl2.style.top = '-20px';
-          targetEl2.style.left = '0px';
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(2);
-            expect(records[0].intersectionRatio).to.be(0);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].isIntersecting).to.be(false);
-            expect(records[1].intersectionRatio).to.be(0);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].isIntersecting).to.be(true);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.top = '0px';
-          targetEl1.style.left = '-20px';
-          targetEl2.style.top = '-21px';
-          targetEl2.style.left = '0px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(2);
-            expect(records[0].intersectionRatio).to.be(0);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[1].intersectionRatio).to.be(0);
-            expect(records[1].target).to.be(targetEl2);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.top = '-20px';
-          targetEl1.style.left = '200px';
-          targetEl2.style.top = '200px';
-          targetEl2.style.left = '200px';
-
-          setTimeout(function() {
-            expect(spy.callCount).to.be(3);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(0);
-            expect(records[0].target).to.be(targetEl2);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-
-    });
-
-
-    it('handles zero-size targets within the root coordinate space',
-        function(done) {
-
-      io = new IntersectionObserver(function(records) {
-        expect(records.length).to.be(1);
-        expect(records[0].isIntersecting).to.be(true);
-        expect(records[0].intersectionRatio).to.be(1);
-        done();
-      }, {root: rootEl});
-
-      targetEl1.style.top = '0px';
-      targetEl1.style.left = '0px';
-      targetEl1.style.width = '0px';
-      targetEl1.style.height = '0px';
-      io.observe(targetEl1);
-    });
-
-
-    it('handles elements with display set to none', function(done) {
-
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl});
-
-      runSequence([
-        function(done) {
-          rootEl.style.display = 'none';
-          io.observe(targetEl1);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].isIntersecting).to.be(false);
-            expect(records[0].intersectionRatio).to.be(0);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          rootEl.style.display = 'block';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].isIntersecting).to.be(true);
-            expect(records[0].intersectionRatio).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          parentEl.style.display = 'none';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(3);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].isIntersecting).to.be(false);
-            expect(records[0].intersectionRatio).to.be(0);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          parentEl.style.display = 'block';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(4);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].isIntersecting).to.be(true);
-            expect(records[0].intersectionRatio).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          targetEl1.style.display = 'none';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(5);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].isIntersecting).to.be(false);
-            expect(records[0].intersectionRatio).to.be(0);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-    });
-
-
-    it('handles target elements not yet added to the DOM', function(done) {
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl});
-
-      // targetEl5 is initially not in the DOM. Note that this element must be
-      // created outside of the addFixtures() function to catch the IE11 error
-      // described here: https://github.com/w3c/IntersectionObserver/pull/205
-      var targetEl5 = document.createElement('div');
-      targetEl5.setAttribute('id', 'target5');
-
-      runSequence([
-        function(done) {
-          io.observe(targetEl5);
-          setTimeout(function() {
-            // Initial observe should trigger with no intersections since
-            // targetEl5 is not yet in the DOM.
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].isIntersecting).to.be(false);
-            expect(records[0].intersectionRatio).to.be(0);
-            expect(records[0].target).to.be(targetEl5);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          // Adding targetEl5 inside rootEl should trigger.
-          parentEl.insertBefore(targetEl5, targetEl2);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(1);
-            expect(records[0].target).to.be(targetEl5);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          // Removing an ancestor of targetEl5 should trigger.
-          grandParentEl.parentNode.removeChild(grandParentEl);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(3);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(0);
-            expect(records[0].target).to.be(targetEl5);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          // Adding the previously removed targetEl5 (via grandParentEl)
-          // back directly inside rootEl should trigger.
-          rootEl.appendChild(targetEl5);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(4);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(1);
-            expect(records[0].target).to.be(targetEl5);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          // Removing rootEl should trigger.
-          rootEl.parentNode.removeChild(rootEl);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(5);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].intersectionRatio).to.be(0);
-            expect(records[0].target).to.be(targetEl5);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-    });
-
-
-    if ('attachShadow' in Element.prototype) {
-      it('handles targets in shadow DOM', function(done) {
-        grandParentEl.attachShadow({mode: 'open'});
-        grandParentEl.shadowRoot.appendChild(parentEl);
-
-        io = new IntersectionObserver(function(records) {
-          expect(records.length).to.be(1);
-          expect(records[0].intersectionRatio).to.be(1);
-          done();
-        }, {root: rootEl});
-
-        io.observe(targetEl1);
-      });
-
-      it('handles roots in shadow DOM', function(done) {
-        var shadowRoot = grandParentEl.attachShadow({mode: 'open'});
-
-        shadowRoot.innerHTML =
-        '<style>' +
-          '#slot-parent {' +
-          '  position: relative;' +
-          '  width: 400px;' +
-          '  height: 200px;' +
-          '}' +
-        '</style>' +
-        '<div id="slot-parent"><slot></slot></div>';
-
-        var slotParent = shadowRoot.getElementById('slot-parent');
-
-        io = new IntersectionObserver(function(records) {
-          expect(records.length).to.be(1);
-          expect(records[0].intersectionRatio).to.be(1);
-          done();
-        }, {root: slotParent});
-
-        io.observe(targetEl1);
-      });
-    }
-
-
-    it('handles sub-root element scrolling', function(done) {
-      io = new IntersectionObserver(function(records) {
-        expect(records.length).to.be(1);
-        expect(records[0].intersectionRatio).to.be(1);
-        done();
-      }, {root: rootEl});
-
-      io.observe(targetEl3);
-      setTimeout(function() {
-        parentEl.scrollLeft = 40;
-      }, 0);
-    });
-
-
-    // Only run this test in browsers that support CSS transitions.
-    if ('transform' in document.documentElement.style &&
-        'transition' in document.documentElement.style) {
-
-      it('supports CSS transitions and transforms', function(done) {
-
-        targetEl1.style.top = '220px';
-        targetEl1.style.left = '220px';
-
-        io = new IntersectionObserver(function(records) {
-          expect(records.length).to.be(1);
-          // Chrome's native implementation sometimes incorrectly reports
-          // the intersection ratio as a number > 1.
-          expect(records[0].intersectionRatio >= 1);
-          done();
-        }, {root: rootEl, threshold: [1]});
-
-        // CSS transitions that are slower than the default throttle timeout
-        // require polling to detect, which can be set on a per-instance basis.
-        if (!supportsNativeIntersectionObserver()) {
-          io.POLL_INTERVAL = 100;
-        }
-
-        io.observe(targetEl1);
-        setTimeout(function() {
-          targetEl1.style.transform = 'translateX(-40px) translateY(-40px)';
-        }, 0);
-      });
-    }
-
-
-    it('uses the viewport when no root is specified', function(done) {
-      io = new IntersectionObserver(function(records) {
-        var viewportWidth =
-            document.documentElement.clientWidth || document.body.clientWidth;
-        var viewportHeight =
-            document.documentElement.clientHeight || document.body.clientHeight;
-
-        expect(records.length).to.be(1);
-        expect(records[0].rootBounds.top).to.be(0);
-        expect(records[0].rootBounds.left).to.be(0);
-        expect(records[0].rootBounds.right).to.be(viewportWidth);
-        expect(records[0].rootBounds.width).to.be(viewportWidth);
-        expect(records[0].rootBounds.bottom).to.be(viewportHeight);
-        expect(records[0].rootBounds.height).to.be(viewportHeight);
-        done();
-      });
-
-      // Ensures targetEl1 is visible in the viewport before observing.
-      window.scrollTo(0, 0);
-      rootEl.style.position = 'absolute';
-      rootEl.style.top = '0px';
-      rootEl.style.left = '0px';
-
-      io.observe(targetEl1);
-    });
-
-  });
-
-
-  describe('takeRecords', function() {
-
-    it('supports getting records before the callback is invoked',
-        function(done) {
-
-      var lastestRecords = [];
-      io = new IntersectionObserver(function(records) {
-        lastestRecords = lastestRecords.concat(records);
-      }, {root: rootEl});
-      io.observe(targetEl1);
-
-      window.requestAnimationFrame && requestAnimationFrame(function() {
-        lastestRecords = lastestRecords.concat(io.takeRecords());
-      });
-
-      setTimeout(function() {
-        expect(lastestRecords.length).to.be(1);
-        expect(lastestRecords[0].intersectionRatio).to.be(1);
-        done();
-      }, ASYNC_TIMEOUT);
-    });
-
-  });
-
-
-  describe('unobserve', function() {
-
-    it('removes targets from the internal store', function(done) {
-
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl});
-
-      runSequence([
-        function(done) {
-          targetEl1.style.top = targetEl2.style.top = '0px';
-          targetEl1.style.left = targetEl2.style.left = '0px';
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(2);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(1);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          io.unobserve(targetEl1);
-          targetEl1.style.top = targetEl2.style.top = '0px';
-          targetEl1.style.left = targetEl2.style.left = '-40px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(1);
-            expect(records[0].target).to.be(targetEl2);
-            expect(records[0].intersectionRatio).to.be(0);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          io.unobserve(targetEl2);
-          targetEl1.style.top = targetEl2.style.top = '0px';
-          targetEl1.style.left = targetEl2.style.left = '0px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(2);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-
-    });
-
-  });
-
-  describe('disconnect', function() {
-
-    it('removes all targets and stops listening for changes', function(done) {
-
-      var spy = sinon.spy();
-      io = new IntersectionObserver(spy, {root: rootEl});
-
-      runSequence([
-        function(done) {
-          targetEl1.style.top = targetEl2.style.top = '0px';
-          targetEl1.style.left = targetEl2.style.left = '0px';
-          io.observe(targetEl1);
-          io.observe(targetEl2);
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            var records = sortRecords(spy.lastCall.args[0]);
-            expect(records.length).to.be(2);
-            expect(records[0].target).to.be(targetEl1);
-            expect(records[0].intersectionRatio).to.be(1);
-            expect(records[1].target).to.be(targetEl2);
-            expect(records[1].intersectionRatio).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        },
-        function(done) {
-          io.disconnect();
-          targetEl1.style.top = targetEl2.style.top = '0px';
-          targetEl1.style.left = targetEl2.style.left = '-40px';
-          setTimeout(function() {
-            expect(spy.callCount).to.be(1);
-            done();
-          }, ASYNC_TIMEOUT);
-        }
-      ], done);
-
-    });
-
-  });
-
-  describe('iframe', function() {
-    var iframe;
-    var iframeWin, iframeDoc;
-    var documentElement, body;
-    var iframeTargetEl1, iframeTargetEl2;
-    var bodyWidth;
-
-    beforeEach(function(done) {
-      iframe = document.createElement('iframe');
-      iframe.setAttribute('frameborder', '0');
-      iframe.setAttribute('scrolling', 'yes');
-      iframe.style.position = 'fixed';
-      iframe.style.top = '0px';
-      iframe.style.width = '100px';
-      iframe.style.height = '200px';
-      iframe.onerror = function() {
-        done(new Error('iframe initialization failed'));
-      };
-      iframe.onload = function() {
-        iframe.onload = null;
-        iframeWin = iframe.contentWindow;
-        iframeDoc = iframeWin.document;
-        iframeDoc.open();
-        iframeDoc.write('<!DOCTYPE html><html><body>');
-        iframeDoc.write('<style>');
-        iframeDoc.write('body {margin: 0}');
-        iframeDoc.write('.target {height: 200px; margin-bottom: 2px; background: blue;}');
-        iframeDoc.write('</style>');
-        iframeDoc.close();
-
-        // Ensure the documentElement and body are always sorted on top. See
-        // `sortRecords` for more info.
-        documentElement = iframeDoc.documentElement;
-        body = iframeDoc.body;
-        documentElement.id = 'A1';
-        body.id = 'A1';
-
-        function createTarget(id, bg) {
-          var target = iframeDoc.createElement('div');
-          target.id = id;
-          target.className = 'target';
-          target.style.background = bg;
-          iframeDoc.body.appendChild(target);
-          return target;
-        }
-        iframeTargetEl1 = createTarget('target1', 'blue');
-        iframeTargetEl2 = createTarget('target2', 'green');
-        bodyWidth = iframeDoc.body.clientWidth;
-        done();
-      };
-      iframe.src = 'about:blank';
-      rootEl.appendChild(iframe);
-    });
-
-    afterEach(function() {
-      rootEl.removeChild(iframe);
-    });
-
-    function rect(r) {
-      return {
-        y: typeof r.y == 'number' ? r.y : r.top,
-        x: typeof r.x == 'number' ? r.x : r.left,
-        top: r.top,
-        left: r.left,
-        width: r.width != null ? r.width : r.right - r.left,
-        height: r.height != null ? r.height : r.bottom - r.top,
-        right: r.right != null ? r.right : r.left + r.width,
-        bottom: r.bottom != null ? r.bottom : r.top + r.height
-      };
-    }
-
-    function getRootRect(doc) {
-      var html = doc.documentElement;
-      var body = doc.body;
-      return rect({
-        top: 0,
-        left: 0,
-        right: html.clientWidth || body.clientWidth,
-        width: html.clientWidth || body.clientWidth,
-        bottom: html.clientHeight || body.clientHeight,
-        height: html.clientHeight || body.clientHeight
-      });
-    }
-
-    describe('same-origin iframe loaded in the mainframe', function() {
-      it('iframe targets do not intersect with a top root element', function(done) {
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(records[0].isIntersecting).to.be(false);
-          expect(records[1].isIntersecting).to.be(false);
-          done();
-          io.disconnect();
-        }, {root: rootEl});
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('triggers for all targets in top-level root', function(done) {
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(records[0].isIntersecting).to.be(true);
-          expect(records[0].intersectionRatio).to.be(1);
-          expect(records[1].isIntersecting).to.be(false);
-          expect(records[1].intersectionRatio).to.be(0);
-
-          // The rootBounds is for the document's root.
-          expect(records[0].rootBounds.height).to.be(innerHeight);
-
-          done();
-          io.disconnect();
-        });
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('triggers for all targets in iframe-level root', function(done) {
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(records[0].intersectionRatio).to.be(1);
-          expect(records[1].intersectionRatio).to.be(1);
-
-          // The rootBounds is for the document's root.
-          expect(rect(records[0].rootBounds)).
-            to.eql(rect(iframeDoc.body.getBoundingClientRect()));
-
-          done();
-          io.disconnect();
-        }, {root: iframeDoc.body});
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a fully visible frame', function(done) {
-        iframe.style.top = '0px';
-        iframe.style.height = '300px';
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(rect(records[0].rootBounds)).to.eql(getRootRect(document));
-          expect(rect(records[1].rootBounds)).to.eql(getRootRect(document));
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[0].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[0].intersectionRect)).to.eql(clientRect1);
-          expect(records[0].isIntersecting).to.be(true);
-          expect(records[0].intersectionRatio).to.be(1);
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300
-          });
-          expect(rect(records[1].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[1].intersectionRect)).to.eql(intersectRect2);
-          expect(records[1].isIntersecting).to.be(true);
-          expect(records[1].intersectionRatio).to.be.within(0.48, 0.5); // ~0.5
-
-          done();
-          io.disconnect();
-        });
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a fully visible and offset frame', function(done) {
-        iframe.style.top = '10px';
-        iframe.style.height = '300px';
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(rect(records[0].rootBounds)).to.eql(getRootRect(document));
-          expect(rect(records[1].rootBounds)).to.eql(getRootRect(document));
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[0].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[0].intersectionRect)).to.eql(clientRect1);
-          expect(records[0].isIntersecting).to.be(true);
-          expect(records[0].intersectionRatio).to.be(1);
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300
-          });
-          expect(rect(records[1].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[1].intersectionRect)).to.eql(intersectRect2);
-          expect(records[1].isIntersecting).to.be(true);
-          expect(records[1].intersectionRatio).to.be.within(0.48, 0.5); // ~0.5
-
-          done();
-          io.disconnect();
-        });
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a clipped frame on top', function(done) {
-        iframe.style.top = '-10px';
-        iframe.style.height = '300px';
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(rect(records[0].rootBounds)).to.eql(getRootRect(document));
-          expect(rect(records[1].rootBounds)).to.eql(getRootRect(document));
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            left: 0,
-            width: bodyWidth,
-            // Top is clipped.
-            top: 10,
-            height: 200 - 10
-          });
-          expect(rect(records[0].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[0].intersectionRect)).to.eql(intersectRect1);
-          expect(records[0].isIntersecting).to.be(true);
-          expect(records[0].intersectionRatio).to.within(0.94, 0.96); // ~0.95
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300
-          });
-          expect(rect(records[1].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[1].intersectionRect)).to.eql(intersectRect2);
-          expect(records[1].isIntersecting).to.be(true);
-          expect(records[1].intersectionRatio).to.be.within(0.48, 0.5); // ~0.49
-
-          done();
-          io.disconnect();
-        });
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a clipped frame on bottom', function(done) {
-        iframe.style.top = 'auto';
-        iframe.style.bottom = '-10px';
-        iframe.style.height = '300px';
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(rect(records[0].rootBounds)).to.eql(getRootRect(document));
-          expect(rect(records[1].rootBounds)).to.eql(getRootRect(document));
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[0].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[0].intersectionRect)).to.eql(clientRect1);
-          expect(records[0].isIntersecting).to.be(true);
-          expect(records[0].intersectionRatio).to.be(1);
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300 - 10
-          });
-          expect(rect(records[1].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[1].intersectionRect)).to.eql(intersectRect2);
-          expect(records[1].isIntersecting).to.be(true);
-          expect(records[1].intersectionRatio).to.be.within(0.43, 0.45); // ~0.44
-
-          done();
-          io.disconnect();
-        });
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a fully visible frame and scrolled', function(done) {
-        iframe.style.top = '0px';
-        iframe.style.height = '300px';
-        iframeWin.scrollTo(0, 10);
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(rect(records[0].rootBounds)).to.eql(getRootRect(document));
-          expect(rect(records[1].rootBounds)).to.eql(getRootRect(document));
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: -10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            // Height is only for the visible area.
-            height: 200 - 10
-          });
-          expect(rect(records[0].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[0].intersectionRect)).to.eql(intersectRect1);
-          expect(records[0].isIntersecting).to.be(true);
-          expect(records[0].intersectionRatio).to.within(0.94, 0.96); // ~0.95
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300
-          });
-          expect(rect(records[1].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[1].intersectionRect)).to.eql(intersectRect2);
-          expect(records[1].isIntersecting).to.be(true);
-          expect(records[1].intersectionRatio).to.be.within(0.53, 0.55); // ~0.54
-
-          done();
-          io.disconnect();
-        });
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a clipped frame on top and scrolled', function(done) {
-        iframe.style.top = '-10px';
-        iframe.style.height = '300px';
-        iframeWin.scrollTo(0, 10);
-        var io = new IntersectionObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(2);
-          expect(rect(records[0].rootBounds)).to.eql(getRootRect(document));
-          expect(rect(records[1].rootBounds)).to.eql(getRootRect(document));
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: -10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            left: 0,
-            width: bodyWidth,
-            // Top is clipped.
-            top: 10,
-            // The height is less by both: offset and scroll.
-            height: 200 - 10 - 10
-          });
-          expect(rect(records[0].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[0].intersectionRect)).to.eql(intersectRect1);
-          expect(records[0].isIntersecting).to.be(true);
-          expect(records[0].intersectionRatio).to.within(0.89, 0.91); // ~0.9
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300
-          });
-          expect(rect(records[1].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[1].intersectionRect)).to.eql(intersectRect2);
-          expect(records[1].isIntersecting).to.be(true);
-          expect(records[1].intersectionRatio).to.be.within(0.53, 0.55); // ~0.54
-
-          done();
-          io.disconnect();
-        });
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('handles tracking iframe viewport', function(done) {
-        iframe.style.height = '100px';
-        iframe.style.top = '100px';
-        iframeWin.scrollTo(0, 110);
-        // {root:iframeDoc} means to track the iframe viewport.
-        var io = new IntersectionObserver(
-          function (records) {
-            io.unobserve(iframeTargetEl1);
-
-            var intersectionRect = rect({
-              top: 0, // if root=null, then this would be 100.
-              left: 0,
-              height: 90,
-              width: bodyWidth
-            });
-            expect(records.length).to.be(1);
-            expect(rect(records[0].rootBounds)).to.eql(getRootRect(iframeDoc));
-            expect(rect(records[0].intersectionRect)).to.eql(intersectionRect);
-            done();
-          },
-          { root: iframeDoc }
-        );
-
-        io.observe(iframeTargetEl1);
-      });
-
-      it('handles tracking iframe viewport with rootMargin', function(done) {
-        iframe.style.height = '100px';
-
-        var io =  new IntersectionObserver(
-          function (records) {
-            io.unobserve(iframeTargetEl1);
-            var intersectionRect = rect({
-              top: 0, // if root=null, then this would be 100.
-              left: 0,
-              height: 200,
-              width: bodyWidth
-            });
-
-            // rootMargin: 100% --> 3x width + 3x height.
-            var expectedRootBounds = rect({
-              top: -100,
-              left: -bodyWidth,
-              width: bodyWidth * 3,
-              height: 100 * 3
-            });
-            expect(records.length).to.be(1);
-            expect(rect(records[0].rootBounds)).to.eql(expectedRootBounds);
-            expect(rect(records[0].intersectionRect)).to.eql(intersectionRect);
-            done();
-          },
-          { root: iframeDoc, rootMargin: '100%' }
-        );
-
-        io.observe(iframeTargetEl1);
-      });
-
-      // Current spec indicates that cross-document tracking yields
-      // an essentially empty IntersectionObserverEntry.
-      // See: https://github.com/w3c/IntersectionObserver/issues/87
-      it('does not track cross-document elements', function(done) {
-        var io = new IntersectionObserver(
-          function (records) {
-            io.unobserve(iframeTargetEl1)
-
-            expect(records.length).to.be(1);
-            const zeroesRect = rect({
-              top: 0,
-              left: 0,
-              width: 0,
-              height: 0
-            });
-            expect(rect(records[0].rootBounds)).to.eql(zeroesRect);
-            expect(rect(records[0].intersectionRect)).to.eql(zeroesRect);
-            expect(records.isIntersecting).false;
-            done();
-          },
-          { root: document }
-        );
-
-        io.observe(iframeTargetEl1);
-      });
-
-      it('handles style changes', function(done) {
-        var spy = sinon.spy();
-
-        // When first element becomes invisible, the second element will show.
-        // And in reverse: when the first element becomes visible again, the
-        // second element will disappear.
-        var io = new IntersectionObserver(spy);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeTargetEl1.style.display = 'none';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              expect(records[1].intersectionRatio).to.be(1);
-              expect(records[1].isIntersecting).to.be(true);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeTargetEl1.style.display = '';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles scroll changes', function(done) {
-        var spy = sinon.spy();
-
-        // Scrolling to the middle of the iframe shows the second box and
-        // hides the first.
-        var io = new IntersectionObserver(spy);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeWin.scrollTo(0, 202);
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              expect(records[1].intersectionRatio).to.be(1);
-              expect(records[1].isIntersecting).to.be(true);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeWin.scrollTo(0, 0);
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles iframe changes', function(done) {
-        var spy = sinon.spy();
-
-        // Iframe goes off screen and returns.
-        var io = new IntersectionObserver(spy);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              // Top-level bounds.
-              expect(records[0].rootBounds.height).to.be(innerHeight);
-              expect(records[0].intersectionRect.height).to.be(200);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Completely off screen.
-            iframe.style.top = '-202px';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(1);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              // Top-level bounds.
-              expect(records[0].rootBounds.height).to.be(innerHeight);
-              expect(records[0].intersectionRect.height).to.be(0);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Partially returns.
-            iframe.style.top = '-100px';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(1);
-              expect(records[0].intersectionRatio).to.be.within(0.45, 0.55);
-              expect(records[0].isIntersecting).to.be(true);
-              // Top-level bounds.
-              expect(records[0].rootBounds.height).to.be(innerHeight);
-              expect(records[0].intersectionRect.height / 200).to.be.within(0.45, 0.55);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('continues to monitor until the last target unobserved', function(done) {
-        var spy = sinon.spy();
-
-        // Iframe goes off screen and returns.
-        var io = new IntersectionObserver(spy);
-        io.observe(target1);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              expect(spy.lastCall.args[0].length).to.be(3);
-
-              // Unobserve one from the main context and one from iframe.
-              io.unobserve(target1);
-              io.unobserve(iframeTargetEl2);
-
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Completely off screen.
-            iframe.style.top = '-202px';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              expect(spy.lastCall.args[0].length).to.be(1);
-
-              io.unobserve(iframeTargetEl1);
-
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Partially returns.
-            iframe.style.top = '-100px';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-    });
-
-    describe('same-origin iframe loaded in an iframe', function() {
-      var ASYNC_TIMEOUT = 300;
-
-      beforeEach(function(done) {
-        /* Uncomment these lines to force polyfill inside the iframe.
-        delete iframeWin.IntersectionObserver;
-        delete iframeWin.IntersectionObserverEntry;
-        */
-
-        // Install polyfill right into the iframe.
-        if (!iframeWin.IntersectionObserver) {
-          var script = iframeDoc.createElement('script');
-          script.src = 'intersection-observer.js';
-          script.onload = function() {
-            done();
-          };
-          iframeDoc.body.appendChild(script);
-        } else {
-          done();
-        }
-      });
-
-      function computeRectIntersection(rect1, rect2) {
-        var top = Math.max(rect1.top, rect2.top);
-        var bottom = Math.min(rect1.bottom, rect2.bottom);
-        var left = Math.max(rect1.left, rect2.left);
-        var right = Math.min(rect1.right, rect2.right);
-        var width = right - left;
-        var height = bottom - top;
-
-        return (width >= 0 && height >= 0) && {
-          top: top,
-          bottom: bottom,
-          left: left,
-          right: right,
-          width: width,
-          height: height
-        } || {
-          top: 0,
-          bottom: 0,
-          left: 0,
-          right: 0,
-          width: 0,
-          height: 0
-        };
-      }
-
-      function checkRootBounds(records) {
-        if (!supportsNativeIntersectionObserver(iframeWin)) {
-          records.forEach(function(record) {
-            expect(rect(record.rootBounds)).to.eql(getRootRect(document));
-          });
-        }
-      }
-
-      function applyParentRect(parentRect) {
-        iframe.style.top = parentRect.top + 'px';
-        iframe.style.left = parentRect.left + 'px';
-        iframe.style.height = parentRect.height + 'px';
-        iframe.style.width = parentRect.width + 'px';
-      }
-
-      function createObserver(callback, options, parentRect) {
-        var io = new iframeWin.IntersectionObserver(callback, options);
-        if (parentRect) {
-          applyParentRect(parentRect);
-        }
-        return io;
-      }
-
-      it('calculates rects for a fully visible frame', function(done) {
-        var parentRect = rect({top: 0, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBounds(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // 300 / 404 == ~0.743
-          expect(records[0].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // 300 / 402 == ~0.746
-          expect(records[1].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(clientRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.be(1);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a fully visible and offset frame', function(done) {
-        var parentRect = rect({top: 10, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBounds(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // 300 / 404 == ~0.743
-          expect(records[0].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // 300 / 402 == ~0.746
-          expect(records[1].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(clientRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.be(1);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a clipped frame on top', function(done) {
-        var parentRect = rect({top: -10, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBounds(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // (300 - 10) / 404 == ~0.717
-          expect(records[0].intersectionRatio).to.be.within(0.71, 0.72);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // (300 - 10) / 402 == ~0.721
-          expect(records[1].intersectionRatio).to.be.within(0.72, 0.73);
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            left: 0,
-            width: bodyWidth,
-            // Top is clipped.
-            top: 10,
-            height: 200 - 10
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(intersectRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.within(0.94, 0.96); // ~0.95
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a clipped frame on bottom', function(done) {
-        var rootRect = getRootRect(document);
-        var parentRect = rect({top: rootRect.bottom - 300 + 10, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBounds(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // (300 - 10) / 404 == ~0.717
-          expect(records[0].intersectionRatio).to.be.within(0.71, 0.72);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // (300 - 10) / 402 == ~0.721
-          expect(records[1].intersectionRatio).to.be.within(0.72, 0.73);
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(clientRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.be(1);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a fully visible and scrolled frame', function(done) {
-        iframeWin.scrollTo(0, 10);
-        var parentRect = rect({top: 0, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBounds(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // 300 / 404 == ~0.743
-          expect(records[0].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // 300 / 402 == ~0.746
-          expect(records[1].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: -10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            // Height is only for the visible area.
-            height: 200 - 10
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(intersectRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.within(0.94, 0.96); // ~0.95
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a clipped frame on top and scrolled', function(done) {
-        iframeWin.scrollTo(0, 10);
-        var parentRect = rect({top: -10, left: 0, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(4);
-          checkRootBounds(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // (300 - 10) / 404 == ~0.717
-          expect(records[0].intersectionRatio).to.be.within(0.71, 0.72);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // (300 - 10) / 402 == ~0.721
-          expect(records[1].intersectionRatio).to.be.within(0.72, 0.73);
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: -10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            left: 0,
-            width: bodyWidth,
-            // Top is clipped.
-            top: 10,
-            // The height is less by both: offset and scroll.
-            height: 200 - 10 - 10
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(intersectRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.within(0.89, 0.91); // ~0.9
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300
-          });
-          expect(rect(records[3].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[3].intersectionRect)).to.eql(intersectRect2);
-          expect(records[3].isIntersecting).to.be(true);
-          expect(records[3].intersectionRatio).to.be.within(0.53, 0.55); // ~0.54
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a fully clipped frame', function(done) {
-        var parentRect = rect({top: -400, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBounds(records);
-
-          var emptyRect = rect({
-            top: 0,
-            left: 0,
-            width: 0,
-            height: 0
-          });
-
-          // The documentElement is completely invisible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(emptyRect);
-          expect(records[0].isIntersecting).to.be(false);
-          expect(records[0].intersectionRatio).to.be(0);
-
-          // The document.body is completely invisible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(emptyRect);
-          expect(records[1].isIntersecting).to.be(false);
-          expect(records[1].intersectionRatio).to.be(0);
-
-          // The target1 is completely invisible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(emptyRect);
-          expect(records[2].isIntersecting).to.be(false);
-          expect(records[2].intersectionRatio).to.be(0);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('handles style changes', function(done) {
-        var spy = sinon.spy();
-
-        var parentRect = rect({top: 0, left: 0, height: 200, width: 100});
-
-        // When first element becomes invisible, the second element will show.
-        // And in reverse: when the first element becomes visible again, the
-        // second element will disappear.
-        var io = createObserver(spy, {}, parentRect);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeTargetEl1.style.display = 'none';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              expect(records[1].intersectionRatio).to.be(1);
-              expect(records[1].isIntersecting).to.be(true);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeTargetEl1.style.display = '';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles scroll changes', function(done) {
-        var spy = sinon.spy();
-
-        var parentRect = rect({top: 0, left: 0, height: 200, width: 100});
-
-        // Scrolling to the middle of the iframe shows the second box and
-        // hides the first.
-        var io = createObserver(spy, {}, parentRect);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeWin.scrollTo(0, 202);
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              expect(records[1].intersectionRatio).to.be(1);
-              expect(records[1].isIntersecting).to.be(true);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeWin.scrollTo(0, 0);
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles parent rect changes', function(done) {
-        var spy = sinon.spy();
-
-        var parentRect = rect({top: 0, left: 0, height: 200, width: 100});
-
-        // Iframe goes off screen and returns.
-        var io = createObserver(spy, {}, parentRect);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              checkRootBounds(records);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              // Top-level bounds.
-              expect(records[0].intersectionRect.height).to.be(200);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Completely off screen.
-            applyParentRect(rect({top: -202, left: 0, height: 200, width: 100}));
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(1);
-              checkRootBounds(records);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              // Top-level bounds.
-              expect(records[0].intersectionRect.height).to.be(0);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Partially returns.
-            applyParentRect(rect({top: -100, left: 0, height: 200, width: 100}));
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(1);
-              checkRootBounds(records);
-              expect(records[0].intersectionRatio).to.be.within(0.45, 0.55);
-              expect(records[0].isIntersecting).to.be(true);
-              // Top-level bounds.
-              expect(records[0].intersectionRect.height / 200).to.be.within(0.45, 0.55);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-    });
-
-    describe('cross-origin iframe', function() {
-      var ASYNC_TIMEOUT = 300;
-      var crossOriginUpdater;
-
-      beforeEach(function(done) {
-        Object.defineProperty(iframeWin, 'frameElement', {value: null});
-
-        /* Uncomment these lines to force polyfill inside the iframe.
-        delete iframeWin.IntersectionObserver;
-        delete iframeWin.IntersectionObserverEntry;
-        */
-
-        // Install polyfill right into the iframe.
-        if (!iframeWin.IntersectionObserver) {
-          var script = iframeDoc.createElement('script');
-          script.src = 'intersection-observer.js';
-          script.onload = function() {
-            if (iframeWin.IntersectionObserver._setupCrossOriginUpdater) {
-              crossOriginUpdater = iframeWin.IntersectionObserver._setupCrossOriginUpdater();
-            }
-            done();
-          };
-          iframeDoc.body.appendChild(script);
-        } else {
-          done();
-        }
-      });
-
-      afterEach(function() {
-        if (IntersectionObserver._resetCrossOriginUpdater) {
-          IntersectionObserver._resetCrossOriginUpdater();
-        }
-      });
-
-      function computeRectIntersection(rect1, rect2) {
-        var top = Math.max(rect1.top, rect2.top);
-        var bottom = Math.min(rect1.bottom, rect2.bottom);
-        var left = Math.max(rect1.left, rect2.left);
-        var right = Math.min(rect1.right, rect2.right);
-        var width = right - left;
-        var height = bottom - top;
-
-        return (width >= 0 && height >= 0) && {
-          top: top,
-          bottom: bottom,
-          left: left,
-          right: right,
-          width: width,
-          height: height
-        } || {
-          top: 0,
-          bottom: 0,
-          left: 0,
-          right: 0,
-          width: 0,
-          height: 0
-        };
-      }
-
-      function checkRootBoundsAreNull(records) {
-        if (!supportsNativeIntersectionObserver(iframeWin)) {
-          records.forEach(function(record) {
-            expect(record.rootBounds).to.be(null);
-          });
-        }
-      }
-
-      function applyParentRect(parentRect) {
-        if (crossOriginUpdater) {
-          var parentIntersectionRect = computeRectIntersection(
-              parentRect, getRootRect(document));
-          crossOriginUpdater(parentRect, parentIntersectionRect);
-        } else {
-          iframe.style.top = parentRect.top + 'px';
-          iframe.style.left = parentRect.left + 'px';
-          iframe.style.height = parentRect.height + 'px';
-          iframe.style.width = parentRect.width + 'px';
-        }
-      }
-
-      function createObserver(callback, options, parentRect) {
-        var io = new iframeWin.IntersectionObserver(callback, options);
-        if (parentRect) {
-          applyParentRect(parentRect);
-        }
-        return io;
-      }
-
-      it('calculates rects for a fully visible frame', function(done) {
-        var parentRect = rect({top: 0, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBoundsAreNull(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // 300 / 404 == ~0.743
-          expect(records[0].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // 300 / 402 == ~0.746
-          expect(records[1].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(clientRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.be(1);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a fully visible and offset frame', function(done) {
-        var parentRect = rect({top: 10, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBoundsAreNull(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // 300 / 404 == ~0.743
-          expect(records[0].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // 300 / 402 == ~0.746
-          expect(records[1].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(clientRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.be(1);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a clipped frame on top', function(done) {
-        var parentRect = rect({top: -10, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBoundsAreNull(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // (300 - 10) / 404 == ~0.717
-          expect(records[0].intersectionRatio).to.be.within(0.71, 0.72);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // (300 - 10) / 402 == ~0.721
-          expect(records[1].intersectionRatio).to.be.within(0.72, 0.73);
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            left: 0,
-            width: bodyWidth,
-            // Top is clipped.
-            top: 10,
-            height: 200 - 10
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(intersectRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.within(0.94, 0.96); // ~0.95
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a clipped frame on bottom', function(done) {
-        var rootRect = getRootRect(document);
-        var parentRect = rect({top: rootRect.bottom - 300 + 10, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBoundsAreNull(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // (300 - 10) / 404 == ~0.717
-          expect(records[0].intersectionRatio).to.be.within(0.71, 0.72);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // (300 - 10) / 402 == ~0.721
-          expect(records[1].intersectionRatio).to.be.within(0.72, 0.73);
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(clientRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.be(1);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a fully visible and scrolled frame', function(done) {
-        iframeWin.scrollTo(0, 10);
-        var parentRect = rect({top: 0, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBoundsAreNull(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // 300 / 404 == ~0.743
-          expect(records[0].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 300
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // 300 / 402 == ~0.746
-          expect(records[1].intersectionRatio).to.be.within(0.74, 0.75);
-
-          // The target1 is fully visible.
-          var clientRect1 = rect({
-            top: -10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            // Height is only for the visible area.
-            height: 200 - 10
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(intersectRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.within(0.94, 0.96); // ~0.95
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('calculates rects for a clipped frame on top and scrolled', function(done) {
-        iframeWin.scrollTo(0, 10);
-        var parentRect = rect({top: -10, left: 0, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(4);
-          checkRootBoundsAreNull(records);
-
-          // The documentElement is partially visible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[0].isIntersecting).to.be(true);
-          // (300 - 10) / 404 == ~0.717
-          expect(records[0].intersectionRatio).to.be.within(0.71, 0.72);
-
-          // The document.body is partially visible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(rect({
-            top: 10,
-            left: 0,
-            width: bodyWidth,
-            height: 300 - 10
-          }));
-          expect(records[1].isIntersecting).to.be(true);
-          // (300 - 10) / 402 == ~0.721
-          expect(records[1].intersectionRatio).to.be.within(0.72, 0.73);
-
-          // The target1 is clipped at the top by the iframe's clipping.
-          var clientRect1 = rect({
-            top: -10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect1 = rect({
-            left: 0,
-            width: bodyWidth,
-            // Top is clipped.
-            top: 10,
-            // The height is less by both: offset and scroll.
-            height: 200 - 10 - 10
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(intersectRect1);
-          expect(records[2].isIntersecting).to.be(true);
-          expect(records[2].intersectionRatio).to.within(0.89, 0.91); // ~0.9
-
-          // The target2 is partially visible.
-          var clientRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          var intersectRect2 = rect({
-            top: 202 - 10,
-            left: 0,
-            width: bodyWidth,
-            // The bottom is clipped off.
-            bottom: 300
-          });
-          expect(rect(records[3].boundingClientRect)).to.eql(clientRect2);
-          expect(rect(records[3].intersectionRect)).to.eql(intersectRect2);
-          expect(records[3].isIntersecting).to.be(true);
-          expect(records[3].intersectionRatio).to.be.within(0.53, 0.55); // ~0.54
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-      });
-
-      it('calculates rects for a fully clipped frame', function(done) {
-        var parentRect = rect({top: -400, left: 20, height: 300, width: 100});
-        var io = createObserver(function(unsortedRecords) {
-          var records = sortRecords(unsortedRecords);
-          expect(records.length).to.be(3);
-          checkRootBoundsAreNull(records);
-
-          var emptyRect = rect({
-            top: 0,
-            left: 0,
-            width: 0,
-            height: 0
-          });
-
-          // The documentElement is completely invisible.
-          expect(rect(records[0].boundingClientRect))
-              .to.eql(rect(documentElement.getBoundingClientRect()));
-          expect(rect(records[0].intersectionRect)).to.eql(emptyRect);
-          expect(records[0].isIntersecting).to.be(false);
-          expect(records[0].intersectionRatio).to.be(0);
-
-          // The document.body is completely invisible.
-          expect(rect(records[1].boundingClientRect))
-              .to.eql(rect(body.getBoundingClientRect()));
-          expect(rect(records[1].intersectionRect)).to.eql(emptyRect);
-          expect(records[1].isIntersecting).to.be(false);
-          expect(records[1].intersectionRatio).to.be(0);
-
-          // The target1 is completely invisible.
-          var clientRect1 = rect({
-            top: 0,
-            left: 0,
-            width: bodyWidth,
-            height: 200
-          });
-          expect(rect(records[2].boundingClientRect)).to.eql(clientRect1);
-          expect(rect(records[2].intersectionRect)).to.eql(emptyRect);
-          expect(records[2].isIntersecting).to.be(false);
-          expect(records[2].intersectionRatio).to.be(0);
-
-          done();
-          io.disconnect();
-        }, {}, parentRect);
-        io.observe(documentElement);
-        io.observe(body);
-        io.observe(iframeTargetEl1);
-      });
-
-      it('blocks until crossOriginUpdater is called first time', function(done) {
-        if (supportsNativeIntersectionObserver(iframeWin)) {
-          // Skip: not possible to emulate with the native observer.
-          done();
-          return;
-        }
-
-        var spy = sinon.spy();
-
-        var parentRect = rect({top: 0, left: 20, height: 300, width: 100});
-
-        var io = createObserver(spy, {});
-        io.observe(iframeTargetEl1);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(0);
-
-              // Issue the first update.
-              crossOriginUpdater(parentRect, null);
-
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(1);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('doesn\'t block with a root specified', function(done) {
-        var spy = sinon.spy();
-
-        var io = createObserver(spy, {root: body});
-        io.observe(iframeTargetEl1);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var record = sortRecords(spy.lastCall.args[0])[0];
-              expect(record.intersectionRatio).to.be(1);
-              expect(record.isIntersecting).to.be(true);
-              expect(rect(record.rootBounds)).to.eql(rect(body.getBoundingClientRect()));
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles style changes', function(done) {
-        var spy = sinon.spy();
-
-        var parentRect = rect({top: 0, left: 0, height: 200, width: 100});
-
-        // When first element becomes invisible, the second element will show.
-        // And in reverse: when the first element becomes visible again, the
-        // second element will disappear.
-        var io = createObserver(spy, {}, parentRect);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeTargetEl1.style.display = 'none';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              expect(records[1].intersectionRatio).to.be(1);
-              expect(records[1].isIntersecting).to.be(true);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeTargetEl1.style.display = '';
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles scroll changes', function(done) {
-        var spy = sinon.spy();
-
-        var parentRect = rect({top: 0, left: 0, height: 200, width: 100});
-
-        // Scrolling to the middle of the iframe shows the second box and
-        // hides the first.
-        var io = createObserver(spy, {}, parentRect);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeWin.scrollTo(0, 202);
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              expect(records[1].intersectionRatio).to.be(1);
-              expect(records[1].isIntersecting).to.be(true);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            iframeWin.scrollTo(0, 0);
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles parent rect changes', function(done) {
-        var spy = sinon.spy();
-
-        var parentRect = rect({top: 0, left: 0, height: 200, width: 100});
-
-        // Iframe goes off screen and returns.
-        var io = createObserver(spy, {}, parentRect);
-        io.observe(iframeTargetEl1);
-        io.observe(iframeTargetEl2);
-
-        runSequence([
-          function(done) {
-            setTimeout(function() {
-              expect(spy.callCount).to.be(1);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(2);
-              checkRootBoundsAreNull(records);
-              expect(records[0].intersectionRatio).to.be(1);
-              expect(records[0].isIntersecting).to.be(true);
-              expect(records[1].intersectionRatio).to.be(0);
-              expect(records[1].isIntersecting).to.be(false);
-              // Top-level bounds.
-              expect(records[0].intersectionRect.height).to.be(200);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Completely off screen.
-            applyParentRect(rect({top: -202, left: 0, height: 200, width: 100}));
-            setTimeout(function() {
-              expect(spy.callCount).to.be(2);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(1);
-              checkRootBoundsAreNull(records);
-              expect(records[0].intersectionRatio).to.be(0);
-              expect(records[0].isIntersecting).to.be(false);
-              // Top-level bounds.
-              expect(records[0].intersectionRect.height).to.be(0);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            // Partially returns.
-            applyParentRect(rect({top: -100, left: 0, height: 200, width: 100}));
-            setTimeout(function() {
-              expect(spy.callCount).to.be(3);
-              var records = sortRecords(spy.lastCall.args[0]);
-              expect(records.length).to.be(1);
-              checkRootBoundsAreNull(records);
-              expect(records[0].intersectionRatio).to.be.within(0.45, 0.55);
-              expect(records[0].isIntersecting).to.be(true);
-              // Top-level bounds.
-              expect(records[0].intersectionRect.height / 200).to.be.within(0.45, 0.55);
-              done();
-            }, ASYNC_TIMEOUT);
-          },
-          function(done) {
-            io.disconnect();
-            done();
-          }
-        ], done);
-      });
-
-      it('handles tracking iframe viewport', function(done) {
-        iframe.style.height = '100px';
-        iframe.style.top = '100px';
-        iframeWin.scrollTo(0, 110);
-        // {root:iframeDoc} means to track the iframe viewport.
-        var io = createObserver(
-          function (records) {
-            io.unobserve(iframeTargetEl1);
-            var intersectionRect = rect({
-              top: 0, // if root=null, then this would be 100.
-              left: 0,
-              height: 90,
-              width: bodyWidth
-            });
-            expect(records.length).to.be(1);
-            expect(rect(records[0].rootBounds)).to.eql(getRootRect(iframeDoc));
-            expect(rect(records[0].intersectionRect)).to.eql(intersectionRect);
-            done();
-          },
-          { root: iframeDoc }
-        );
-
-        io.observe(iframeTargetEl1);
-      });
-
-      it('handles tracking iframe viewport with rootMargin', function(done) {
-        iframe.style.height = '100px';
-
-        var io = createObserver(
-          function (records) {
-            io.unobserve(iframeTargetEl1);
-            var intersectionRect = rect({
-              top: 0, // if root=null, then this would be 100.
-              left: 0,
-              height: 200,
-              width: bodyWidth
-            });
-
-            // rootMargin: 100% --> 3x width + 3x height.
-            var expectedRootBounds = rect({
-              top: -100,
-              left: -bodyWidth,
-              width: bodyWidth * 3,
-              height: 100 * 3
-            });
-            expect(records.length).to.be(1);
-            expect(rect(records[0].rootBounds)).to.eql(expectedRootBounds);
-            expect(rect(records[0].intersectionRect)).to.eql(intersectionRect);
-            done();
-          },
-          { root: iframeDoc, rootMargin: '100%' }
-        );
-
-        io.observe(iframeTargetEl1);
-      });
-    });
-  });
-});
-
-
-/**
- * Runs a sequence of function and when finished invokes the done callback.
- * Each function in the sequence is invoked with its own done function and
- * it should call that function once it's complete.
- * @param {Array<Function>} functions An array of async functions.
- * @param {Function} done A final callback to be invoked once all function
- *     have run.
- */
-function runSequence(functions, done) {
-  var next = functions.shift();
-  if (next) {
-    next(function() {
-      runSequence(functions, done);
-    });
-  } else {
-    done && done();
-  }
-}
-
-
-/**
- * Returns whether or not the current browser has native support for
- * IntersectionObserver.
- * @param {Window=} win
- * @return {boolean} True if native support is detected.
- */
-function supportsNativeIntersectionObserver(win) {
-  win = win || window;
-  return 'IntersectionObserver' in win &&
-      win.IntersectionObserver.toString().indexOf('[native code]') > -1;
-}
-
-
-/**
- * Sorts an array of records alphebetically by ascending ID. Since the current
- * native implementation doesn't sort change entries by `observe` order, we do
- * that ourselves for the non-polyfill case. Since all tests call observe
- * on targets in sequential order, this should always match.
- * https://crbug.com/613679
- * @param {Array<IntersectionObserverEntry>} entries The entries to sort.
- * @return {Array<IntersectionObserverEntry>} The sorted array.
- */
-function sortRecords(entries) {
-  if (supportsNativeIntersectionObserver()) {
-    entries = entries.sort(function(a, b) {
-      return a.target.id < b.target.id ? -1 : 1;
-    });
-  }
-  return entries;
-}
-
-
-/**
- * Adds the common styles used by all tests to the page.
- */
-function addStyles() {
-  var styles = document.createElement('style');
-  styles.id = 'styles';
-  document.documentElement.appendChild(styles);
-
-  var cssText =
-      '#root {' +
-      '  position: relative;' +
-      '  width: 400px;' +
-      '  height: 200px;' +
-      '  background: #eee' +
-      '}' +
-      '#grand-parent {' +
-      '  position: relative;' +
-      '  width: 200px;' +
-      '  height: 200px;' +
-      '}' +
-      '#parent {' +
-      '  position: absolute;' +
-      '  top: 0px;' +
-      '  left: 200px;' +
-      '  overflow: hidden;' +
-      '  width: 200px;' +
-      '  height: 200px;' +
-      '  background: #ddd;' +
-      '}' +
-      '#target1, #target2, #target3, #target4, #target5 {' +
-      '  position: absolute;' +
-      '  top: 0px;' +
-      '  left: 0px;' +
-      '  width: 20px;' +
-      '  height: 20px;' +
-      '  transform: translateX(0px) translateY(0px);' +
-      '  transition: transform .5s;' +
-      '  background: #f00;' +
-      '}';
-
-  // IE8 doesn't allow setting innerHTML on a <style> element.
-  if (styles.styleSheet) {
-    styles.styleSheet.cssText = cssText;
-  }
-  else {
-    styles.innerHTML = cssText;
-  }
-}
-
-
-/**
- * Adds the DOM fixtures used by all tests to the page and assigns them to
- * global variables so they can be referenced within the tests.
- */
-function addFixtures() {
-  var fixtures = document.createElement('div');
-  fixtures.id = 'fixtures';
-
-  fixtures.innerHTML =
-      '<div id="root">' +
-      '  <div id="grand-parent">' +
-      '    <div id="parent">' +
-      '      <div id="target1"></div>' +
-      '      <div id="target2"></div>' +
-      '      <div id="target3"></div>' +
-      '      <div id="target4"></div>' +
-      '    </div>' +
-      '  </div>' +
-      '</div>';
-
-  document.body.appendChild(fixtures);
-
-  rootEl = document.getElementById('root');
-  grandParentEl = document.getElementById('grand-parent');
-  parentEl = document.getElementById('parent');
-  targetEl1 = document.getElementById('target1');
-  targetEl2 = document.getElementById('target2');
-  targetEl3 = document.getElementById('target3');
-  targetEl4 = document.getElementById('target4');
-}
-
-
-/**
- * Removes the common styles from the page.
- */
-function removeStyles() {
-  var styles = document.getElementById('styles');
-  styles.parentNode.removeChild(styles);
-}
-
-
-/**
- * Removes the DOM fixtures from the page and resets the global references.
- */
-function removeFixtures() {
-  var fixtures = document.getElementById('fixtures');
-  fixtures.parentNode.removeChild(fixtures);
-
-  rootEl = null;
-  grandParentEl = null;
-  parentEl = null;
-  targetEl1 = null;
-  targetEl2 = null;
-  targetEl3 = null;
-  targetEl4 = null;
-}
diff --git a/polyfill/intersection-observer.js b/polyfill/intersection-observer.js
deleted file mode 100644
index 9b7a847..0000000
--- a/polyfill/intersection-observer.js
+++ /dev/null
@@ -1,1014 +0,0 @@
-/**
- * Copyright 2016 Google Inc. All Rights Reserved.
- *
- * Licensed under the W3C SOFTWARE AND DOCUMENT NOTICE AND LICENSE.
- *
- *  https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document
- *
- */
-(function() {
-'use strict';
-
-// Exit early if we're not running in a browser.
-if (typeof window !== 'object') {
-  return;
-}
-
-// Exit early if all IntersectionObserver and IntersectionObserverEntry
-// features are natively supported.
-if ('IntersectionObserver' in window &&
-    'IntersectionObserverEntry' in window &&
-    'intersectionRatio' in window.IntersectionObserverEntry.prototype) {
-
-  // Minimal polyfill for Edge 15's lack of `isIntersecting`
-  // See: https://github.com/w3c/IntersectionObserver/issues/211
-  if (!('isIntersecting' in window.IntersectionObserverEntry.prototype)) {
-    Object.defineProperty(window.IntersectionObserverEntry.prototype,
-      'isIntersecting', {
-      get: function () {
-        return this.intersectionRatio > 0;
-      }
-    });
-  }
-  return;
-}
-
-/**
- * Returns the embedding frame element, if any.
- * @param {!Document} doc
- * @return {!Element}
- */
-function getFrameElement(doc) {
-  try {
-    return doc.defaultView && doc.defaultView.frameElement || null;
-  } catch (e) {
-    // Ignore the error.
-    return null;
-  }
-}
-
-/**
- * A local reference to the root document.
- */
-var document = (function(startDoc) {
-  var doc = startDoc;
-  var frame = getFrameElement(doc);
-  while (frame) {
-    doc = frame.ownerDocument;
-    frame = getFrameElement(doc);
-  }
-  return doc;
-})(window.document);
-
-/**
- * An IntersectionObserver registry. This registry exists to hold a strong
- * reference to IntersectionObserver instances currently observing a target
- * element. Without this registry, instances without another reference may be
- * garbage collected.
- */
-var registry = [];
-
-/**
- * The signal updater for cross-origin intersection. When not null, it means
- * that the polyfill is configured to work in a cross-origin mode.
- * @type {function(DOMRect|ClientRect, DOMRect|ClientRect)}
- */
-var crossOriginUpdater = null;
-
-/**
- * The current cross-origin intersection. Only used in the cross-origin mode.
- * @type {DOMRect|ClientRect}
- */
-var crossOriginRect = null;
-
-
-/**
- * Creates the global IntersectionObserverEntry constructor.
- * https://w3c.github.io/IntersectionObserver/#intersection-observer-entry
- * @param {Object} entry A dictionary of instance properties.
- * @constructor
- */
-function IntersectionObserverEntry(entry) {
-  this.time = entry.time;
-  this.target = entry.target;
-  this.rootBounds = ensureDOMRect(entry.rootBounds);
-  this.boundingClientRect = ensureDOMRect(entry.boundingClientRect);
-  this.intersectionRect = ensureDOMRect(entry.intersectionRect || getEmptyRect());
-  this.isIntersecting = !!entry.intersectionRect;
-
-  // Calculates the intersection ratio.
-  var targetRect = this.boundingClientRect;
-  var targetArea = targetRect.width * targetRect.height;
-  var intersectionRect = this.intersectionRect;
-  var intersectionArea = intersectionRect.width * intersectionRect.height;
-
-  // Sets intersection ratio.
-  if (targetArea) {
-    // Round the intersection ratio to avoid floating point math issues:
-    // https://github.com/w3c/IntersectionObserver/issues/324
-    this.intersectionRatio = Number((intersectionArea / targetArea).toFixed(4));
-  } else {
-    // If area is zero and is intersecting, sets to 1, otherwise to 0
-    this.intersectionRatio = this.isIntersecting ? 1 : 0;
-  }
-}
-
-
-/**
- * Creates the global IntersectionObserver constructor.
- * https://w3c.github.io/IntersectionObserver/#intersection-observer-interface
- * @param {Function} callback The function to be invoked after intersection
- *     changes have queued. The function is not invoked if the queue has
- *     been emptied by calling the `takeRecords` method.
- * @param {Object=} opt_options Optional configuration options.
- * @constructor
- */
-function IntersectionObserver(callback, opt_options) {
-
-  var options = opt_options || {};
-
-  if (typeof callback != 'function') {
-    throw new Error('callback must be a function');
-  }
-
-  if (
-    options.root &&
-    options.root.nodeType != 1 &&
-    options.root.nodeType != 9
-  ) {
-    throw new Error('root must be a Document or Element');
-  }
-
-  // Binds and throttles `this._checkForIntersections`.
-  this._checkForIntersections = throttle(
-      this._checkForIntersections.bind(this), this.THROTTLE_TIMEOUT);
-
-  // Private properties.
-  this._callback = callback;
-  this._observationTargets = [];
-  this._queuedEntries = [];
-  this._rootMarginValues = this._parseRootMargin(options.rootMargin);
-
-  // Public properties.
-  this.thresholds = this._initThresholds(options.threshold);
-  this.root = options.root || null;
-  this.rootMargin = this._rootMarginValues.map(function(margin) {
-    return margin.value + margin.unit;
-  }).join(' ');
-
-  /** @private @const {!Array<!Document>} */
-  this._monitoringDocuments = [];
-  /** @private @const {!Array<function()>} */
-  this._monitoringUnsubscribes = [];
-}
-
-
-/**
- * The minimum interval within which the document will be checked for
- * intersection changes.
- */
-IntersectionObserver.prototype.THROTTLE_TIMEOUT = 100;
-
-
-/**
- * The frequency in which the polyfill polls for intersection changes.
- * this can be updated on a per instance basis and must be set prior to
- * calling `observe` on the first target.
- */
-IntersectionObserver.prototype.POLL_INTERVAL = null;
-
-/**
- * Use a mutation observer on the root element
- * to detect intersection changes.
- */
-IntersectionObserver.prototype.USE_MUTATION_OBSERVER = true;
-
-
-/**
- * Sets up the polyfill in the cross-origin mode. The result is the
- * updater function that accepts two arguments: `boundingClientRect` and
- * `intersectionRect` - just as these fields would be available to the
- * parent via `IntersectionObserverEntry`. This function should be called
- * each time the iframe receives intersection information from the parent
- * window, e.g. via messaging.
- * @return {function(DOMRect|ClientRect, DOMRect|ClientRect)}
- */
-IntersectionObserver._setupCrossOriginUpdater = function() {
-  if (!crossOriginUpdater) {
-    /**
-     * @param {DOMRect|ClientRect} boundingClientRect
-     * @param {DOMRect|ClientRect} intersectionRect
-     */
-    crossOriginUpdater = function(boundingClientRect, intersectionRect) {
-      if (!boundingClientRect || !intersectionRect) {
-        crossOriginRect = getEmptyRect();
-      } else {
-        crossOriginRect = convertFromParentRect(boundingClientRect, intersectionRect);
-      }
-      registry.forEach(function(observer) {
-        observer._checkForIntersections();
-      });
-    };
-  }
-  return crossOriginUpdater;
-};
-
-
-/**
- * Resets the cross-origin mode.
- */
-IntersectionObserver._resetCrossOriginUpdater = function() {
-  crossOriginUpdater = null;
-  crossOriginRect = null;
-};
-
-
-/**
- * Starts observing a target element for intersection changes based on
- * the thresholds values.
- * @param {Element} target The DOM element to observe.
- */
-IntersectionObserver.prototype.observe = function(target) {
-  var isTargetAlreadyObserved = this._observationTargets.some(function(item) {
-    return item.element == target;
-  });
-
-  if (isTargetAlreadyObserved) {
-    return;
-  }
-
-  if (!(target && target.nodeType == 1)) {
-    throw new Error('target must be an Element');
-  }
-
-  this._registerInstance();
-  this._observationTargets.push({element: target, entry: null});
-  this._monitorIntersections(target.ownerDocument);
-  this._checkForIntersections();
-};
-
-
-/**
- * Stops observing a target element for intersection changes.
- * @param {Element} target The DOM element to observe.
- */
-IntersectionObserver.prototype.unobserve = function(target) {
-  this._observationTargets =
-      this._observationTargets.filter(function(item) {
-        return item.element != target;
-      });
-  this._unmonitorIntersections(target.ownerDocument);
-  if (this._observationTargets.length == 0) {
-    this._unregisterInstance();
-  }
-};
-
-
-/**
- * Stops observing all target elements for intersection changes.
- */
-IntersectionObserver.prototype.disconnect = function() {
-  this._observationTargets = [];
-  this._unmonitorAllIntersections();
-  this._unregisterInstance();
-};
-
-
-/**
- * Returns any queue entries that have not yet been reported to the
- * callback and clears the queue. This can be used in conjunction with the
- * callback to obtain the absolute most up-to-date intersection information.
- * @return {Array} The currently queued entries.
- */
-IntersectionObserver.prototype.takeRecords = function() {
-  var records = this._queuedEntries.slice();
-  this._queuedEntries = [];
-  return records;
-};
-
-
-/**
- * Accepts the threshold value from the user configuration object and
- * returns a sorted array of unique threshold values. If a value is not
- * between 0 and 1 and error is thrown.
- * @private
- * @param {Array|number=} opt_threshold An optional threshold value or
- *     a list of threshold values, defaulting to [0].
- * @return {Array} A sorted list of unique and valid threshold values.
- */
-IntersectionObserver.prototype._initThresholds = function(opt_threshold) {
-  var threshold = opt_threshold || [0];
-  if (!Array.isArray(threshold)) threshold = [threshold];
-
-  return threshold.sort().filter(function(t, i, a) {
-    if (typeof t != 'number' || isNaN(t) || t < 0 || t > 1) {
-      throw new Error('threshold must be a number between 0 and 1 inclusively');
-    }
-    return t !== a[i - 1];
-  });
-};
-
-
-/**
- * Accepts the rootMargin value from the user configuration object
- * and returns an array of the four margin values as an object containing
- * the value and unit properties. If any of the values are not properly
- * formatted or use a unit other than px or %, and error is thrown.
- * @private
- * @param {string=} opt_rootMargin An optional rootMargin value,
- *     defaulting to '0px'.
- * @return {Array<Object>} An array of margin objects with the keys
- *     value and unit.
- */
-IntersectionObserver.prototype._parseRootMargin = function(opt_rootMargin) {
-  var marginString = opt_rootMargin || '0px';
-  var margins = marginString.split(/\s+/).map(function(margin) {
-    var parts = /^(-?\d*\.?\d+)(px|%)$/.exec(margin);
-    if (!parts) {
-      throw new Error('rootMargin must be specified in pixels or percent');
-    }
-    return {value: parseFloat(parts[1]), unit: parts[2]};
-  });
-
-  // Handles shorthand.
-  margins[1] = margins[1] || margins[0];
-  margins[2] = margins[2] || margins[0];
-  margins[3] = margins[3] || margins[1];
-
-  return margins;
-};
-
-
-/**
- * Starts polling for intersection changes if the polling is not already
- * happening, and if the page's visibility state is visible.
- * @param {!Document} doc
- * @private
- */
-IntersectionObserver.prototype._monitorIntersections = function(doc) {
-  var win = doc.defaultView;
-  if (!win) {
-    // Already destroyed.
-    return;
-  }
-  if (this._monitoringDocuments.indexOf(doc) != -1) {
-    // Already monitoring.
-    return;
-  }
-
-  // Private state for monitoring.
-  var callback = this._checkForIntersections;
-  var monitoringInterval = null;
-  var domObserver = null;
-
-  // If a poll interval is set, use polling instead of listening to
-  // resize and scroll events or DOM mutations.
-  if (this.POLL_INTERVAL) {
-    monitoringInterval = win.setInterval(callback, this.POLL_INTERVAL);
-  } else {
-    addEvent(win, 'resize', callback, true);
-    addEvent(doc, 'scroll', callback, true);
-    if (this.USE_MUTATION_OBSERVER && 'MutationObserver' in win) {
-      domObserver = new win.MutationObserver(callback);
-      domObserver.observe(doc, {
-        attributes: true,
-        childList: true,
-        characterData: true,
-        subtree: true
-      });
-    }
-  }
-
-  this._monitoringDocuments.push(doc);
-  this._monitoringUnsubscribes.push(function() {
-    // Get the window object again. When a friendly iframe is destroyed, it
-    // will be null.
-    var win = doc.defaultView;
-
-    if (win) {
-      if (monitoringInterval) {
-        win.clearInterval(monitoringInterval);
-      }
-      removeEvent(win, 'resize', callback, true);
-    }
-
-    removeEvent(doc, 'scroll', callback, true);
-    if (domObserver) {
-      domObserver.disconnect();
-    }
-  });
-
-  // Also monitor the parent.
-  var rootDoc =
-    (this.root && (this.root.ownerDocument || this.root)) || document;
-  if (doc != rootDoc) {
-    var frame = getFrameElement(doc);
-    if (frame) {
-      this._monitorIntersections(frame.ownerDocument);
-    }
-  }
-};
-
-
-/**
- * Stops polling for intersection changes.
- * @param {!Document} doc
- * @private
- */
-IntersectionObserver.prototype._unmonitorIntersections = function(doc) {
-  var index = this._monitoringDocuments.indexOf(doc);
-  if (index == -1) {
-    return;
-  }
-
-  var rootDoc =
-    (this.root && (this.root.ownerDocument || this.root)) || document;
-
-  // Check if any dependent targets are still remaining.
-  var hasDependentTargets =
-      this._observationTargets.some(function(item) {
-        var itemDoc = item.element.ownerDocument;
-        // Target is in this context.
-        if (itemDoc == doc) {
-          return true;
-        }
-        // Target is nested in this context.
-        while (itemDoc && itemDoc != rootDoc) {
-          var frame = getFrameElement(itemDoc);
-          itemDoc = frame && frame.ownerDocument;
-          if (itemDoc == doc) {
-            return true;
-          }
-        }
-        return false;
-      });
-  if (hasDependentTargets) {
-    return;
-  }
-
-  // Unsubscribe.
-  var unsubscribe = this._monitoringUnsubscribes[index];
-  this._monitoringDocuments.splice(index, 1);
-  this._monitoringUnsubscribes.splice(index, 1);
-  unsubscribe();
-
-  // Also unmonitor the parent.
-  if (doc != rootDoc) {
-    var frame = getFrameElement(doc);
-    if (frame) {
-      this._unmonitorIntersections(frame.ownerDocument);
-    }
-  }
-};
-
-
-/**
- * Stops polling for intersection changes.
- * @param {!Document} doc
- * @private
- */
-IntersectionObserver.prototype._unmonitorAllIntersections = function() {
-  var unsubscribes = this._monitoringUnsubscribes.slice(0);
-  this._monitoringDocuments.length = 0;
-  this._monitoringUnsubscribes.length = 0;
-  for (var i = 0; i < unsubscribes.length; i++) {
-    unsubscribes[i]();
-  }
-};
-
-
-/**
- * Scans each observation target for intersection changes and adds them
- * to the internal entries queue. If new entries are found, it
- * schedules the callback to be invoked.
- * @private
- */
-IntersectionObserver.prototype._checkForIntersections = function() {
-  if (!this.root && crossOriginUpdater && !crossOriginRect) {
-    // Cross origin monitoring, but no initial data available yet.
-    return;
-  }
-
-  var rootIsInDom = this._rootIsInDom();
-  var rootRect = rootIsInDom ? this._getRootRect() : getEmptyRect();
-
-  this._observationTargets.forEach(function(item) {
-    var target = item.element;
-    var targetRect = getBoundingClientRect(target);
-    var rootContainsTarget = this._rootContainsTarget(target);
-    var oldEntry = item.entry;
-    var intersectionRect = rootIsInDom && rootContainsTarget &&
-        this._computeTargetAndRootIntersection(target, targetRect, rootRect);
-
-    var rootBounds = null;
-    if (!this._rootContainsTarget(target)) {
-      rootBounds = getEmptyRect();
-    } else if (!crossOriginUpdater || this.root) {
-      rootBounds = rootRect;
-    }
-
-    var newEntry = item.entry = new IntersectionObserverEntry({
-      time: now(),
-      target: target,
-      boundingClientRect: targetRect,
-      rootBounds: rootBounds,
-      intersectionRect: intersectionRect
-    });
-
-    if (!oldEntry) {
-      this._queuedEntries.push(newEntry);
-    } else if (rootIsInDom && rootContainsTarget) {
-      // If the new entry intersection ratio has crossed any of the
-      // thresholds, add a new entry.
-      if (this._hasCrossedThreshold(oldEntry, newEntry)) {
-        this._queuedEntries.push(newEntry);
-      }
-    } else {
-      // If the root is not in the DOM or target is not contained within
-      // root but the previous entry for this target had an intersection,
-      // add a new record indicating removal.
-      if (oldEntry && oldEntry.isIntersecting) {
-        this._queuedEntries.push(newEntry);
-      }
-    }
-  }, this);
-
-  if (this._queuedEntries.length) {
-    this._callback(this.takeRecords(), this);
-  }
-};
-
-
-/**
- * Accepts a target and root rect computes the intersection between then
- * following the algorithm in the spec.
- * TODO(philipwalton): at this time clip-path is not considered.
- * https://w3c.github.io/IntersectionObserver/#calculate-intersection-rect-algo
- * @param {Element} target The target DOM element
- * @param {Object} targetRect The bounding rect of the target.
- * @param {Object} rootRect The bounding rect of the root after being
- *     expanded by the rootMargin value.
- * @return {?Object} The final intersection rect object or undefined if no
- *     intersection is found.
- * @private
- */
-IntersectionObserver.prototype._computeTargetAndRootIntersection =
-    function(target, targetRect, rootRect) {
-  // If the element isn't displayed, an intersection can't happen.
-  if (window.getComputedStyle(target).display == 'none') return;
-
-  var intersectionRect = targetRect;
-  var parent = getParentNode(target);
-  var atRoot = false;
-
-  while (!atRoot && parent) {
-    var parentRect = null;
-    var parentComputedStyle = parent.nodeType == 1 ?
-        window.getComputedStyle(parent) : {};
-
-    // If the parent isn't displayed, an intersection can't happen.
-    if (parentComputedStyle.display == 'none') return null;
-
-    if (parent == this.root || parent.nodeType == /* DOCUMENT */ 9) {
-      atRoot = true;
-      if (parent == this.root || parent == document) {
-        if (crossOriginUpdater && !this.root) {
-          if (!crossOriginRect ||
-              crossOriginRect.width == 0 && crossOriginRect.height == 0) {
-            // A 0-size cross-origin intersection means no-intersection.
-            parent = null;
-            parentRect = null;
-            intersectionRect = null;
-          } else {
-            parentRect = crossOriginRect;
-          }
-        } else {
-          parentRect = rootRect;
-        }
-      } else {
-        // Check if there's a frame that can be navigated to.
-        var frame = getParentNode(parent);
-        var frameRect = frame && getBoundingClientRect(frame);
-        var frameIntersect =
-            frame &&
-            this._computeTargetAndRootIntersection(frame, frameRect, rootRect);
-        if (frameRect && frameIntersect) {
-          parent = frame;
-          parentRect = convertFromParentRect(frameRect, frameIntersect);
-        } else {
-          parent = null;
-          intersectionRect = null;
-        }
-      }
-    } else {
-      // If the element has a non-visible overflow, and it's not the <body>
-      // or <html> element, update the intersection rect.
-      // Note: <body> and <html> cannot be clipped to a rect that's not also
-      // the document rect, so no need to compute a new intersection.
-      var doc = parent.ownerDocument;
-      if (parent != doc.body &&
-          parent != doc.documentElement &&
-          parentComputedStyle.overflow != 'visible') {
-        parentRect = getBoundingClientRect(parent);
-      }
-    }
-
-    // If either of the above conditionals set a new parentRect,
-    // calculate new intersection data.
-    if (parentRect) {
-      intersectionRect = computeRectIntersection(parentRect, intersectionRect);
-    }
-    if (!intersectionRect) break;
-    parent = parent && getParentNode(parent);
-  }
-  return intersectionRect;
-};
-
-
-/**
- * Returns the root rect after being expanded by the rootMargin value.
- * @return {ClientRect} The expanded root rect.
- * @private
- */
-IntersectionObserver.prototype._getRootRect = function() {
-  var rootRect;
-  if (this.root && !isDoc(this.root)) {
-    rootRect = getBoundingClientRect(this.root);
-  } else {
-    // Use <html>/<body> instead of window since scroll bars affect size.
-    var doc = isDoc(this.root) ? this.root : document;
-    var html = doc.documentElement;
-    var body = doc.body;
-    rootRect = {
-      top: 0,
-      left: 0,
-      right: html.clientWidth || body.clientWidth,
-      width: html.clientWidth || body.clientWidth,
-      bottom: html.clientHeight || body.clientHeight,
-      height: html.clientHeight || body.clientHeight
-    };
-  }
-  return this._expandRectByRootMargin(rootRect);
-};
-
-
-/**
- * Accepts a rect and expands it by the rootMargin value.
- * @param {DOMRect|ClientRect} rect The rect object to expand.
- * @return {ClientRect} The expanded rect.
- * @private
- */
-IntersectionObserver.prototype._expandRectByRootMargin = function(rect) {
-  var margins = this._rootMarginValues.map(function(margin, i) {
-    return margin.unit == 'px' ? margin.value :
-        margin.value * (i % 2 ? rect.width : rect.height) / 100;
-  });
-  var newRect = {
-    top: rect.top - margins[0],
-    right: rect.right + margins[1],
-    bottom: rect.bottom + margins[2],
-    left: rect.left - margins[3]
-  };
-  newRect.width = newRect.right - newRect.left;
-  newRect.height = newRect.bottom - newRect.top;
-
-  return newRect;
-};
-
-
-/**
- * Accepts an old and new entry and returns true if at least one of the
- * threshold values has been crossed.
- * @param {?IntersectionObserverEntry} oldEntry The previous entry for a
- *    particular target element or null if no previous entry exists.
- * @param {IntersectionObserverEntry} newEntry The current entry for a
- *    particular target element.
- * @return {boolean} Returns true if a any threshold has been crossed.
- * @private
- */
-IntersectionObserver.prototype._hasCrossedThreshold =
-    function(oldEntry, newEntry) {
-
-  // To make comparing easier, an entry that has a ratio of 0
-  // but does not actually intersect is given a value of -1
-  var oldRatio = oldEntry && oldEntry.isIntersecting ?
-      oldEntry.intersectionRatio || 0 : -1;
-  var newRatio = newEntry.isIntersecting ?
-      newEntry.intersectionRatio || 0 : -1;
-
-  // Ignore unchanged ratios
-  if (oldRatio === newRatio) return;
-
-  for (var i = 0; i < this.thresholds.length; i++) {
-    var threshold = this.thresholds[i];
-
-    // Return true if an entry matches a threshold or if the new ratio
-    // and the old ratio are on the opposite sides of a threshold.
-    if (threshold == oldRatio || threshold == newRatio ||
-        threshold < oldRatio !== threshold < newRatio) {
-      return true;
-    }
-  }
-};
-
-
-/**
- * Returns whether or not the root element is an element and is in the DOM.
- * @return {boolean} True if the root element is an element and is in the DOM.
- * @private
- */
-IntersectionObserver.prototype._rootIsInDom = function() {
-  return !this.root || containsDeep(document, this.root);
-};
-
-
-/**
- * Returns whether or not the target element is a child of root.
- * @param {Element} target The target element to check.
- * @return {boolean} True if the target element is a child of root.
- * @private
- */
-IntersectionObserver.prototype._rootContainsTarget = function(target) {
-  var rootDoc =
-    (this.root && (this.root.ownerDocument || this.root)) || document;
-  return (
-    containsDeep(rootDoc, target) &&
-    (!this.root || rootDoc == target.ownerDocument)
-  );
-};
-
-
-/**
- * Adds the instance to the global IntersectionObserver registry if it isn't
- * already present.
- * @private
- */
-IntersectionObserver.prototype._registerInstance = function() {
-  if (registry.indexOf(this) < 0) {
-    registry.push(this);
-  }
-};
-
-
-/**
- * Removes the instance from the global IntersectionObserver registry.
- * @private
- */
-IntersectionObserver.prototype._unregisterInstance = function() {
-  var index = registry.indexOf(this);
-  if (index != -1) registry.splice(index, 1);
-};
-
-
-/**
- * Returns the result of the performance.now() method or null in browsers
- * that don't support the API.
- * @return {number} The elapsed time since the page was requested.
- */
-function now() {
-  return window.performance && performance.now && performance.now();
-}
-
-
-/**
- * Throttles a function and delays its execution, so it's only called at most
- * once within a given time period.
- * @param {Function} fn The function to throttle.
- * @param {number} timeout The amount of time that must pass before the
- *     function can be called again.
- * @return {Function} The throttled function.
- */
-function throttle(fn, timeout) {
-  var timer = null;
-  return function () {
-    if (!timer) {
-      timer = setTimeout(function() {
-        fn();
-        timer = null;
-      }, timeout);
-    }
-  };
-}
-
-
-/**
- * Adds an event handler to a DOM node ensuring cross-browser compatibility.
- * @param {Node} node The DOM node to add the event handler to.
- * @param {string} event The event name.
- * @param {Function} fn The event handler to add.
- * @param {boolean} opt_useCapture Optionally adds the even to the capture
- *     phase. Note: this only works in modern browsers.
- */
-function addEvent(node, event, fn, opt_useCapture) {
-  if (typeof node.addEventListener == 'function') {
-    node.addEventListener(event, fn, opt_useCapture || false);
-  }
-  else if (typeof node.attachEvent == 'function') {
-    node.attachEvent('on' + event, fn);
-  }
-}
-
-
-/**
- * Removes a previously added event handler from a DOM node.
- * @param {Node} node The DOM node to remove the event handler from.
- * @param {string} event The event name.
- * @param {Function} fn The event handler to remove.
- * @param {boolean} opt_useCapture If the event handler was added with this
- *     flag set to true, it should be set to true here in order to remove it.
- */
-function removeEvent(node, event, fn, opt_useCapture) {
-  if (typeof node.removeEventListener == 'function') {
-    node.removeEventListener(event, fn, opt_useCapture || false);
-  }
-  else if (typeof node.detachEvent == 'function') {
-    node.detachEvent('on' + event, fn);
-  }
-}
-
-
-/**
- * Returns the intersection between two rect objects.
- * @param {Object} rect1 The first rect.
- * @param {Object} rect2 The second rect.
- * @return {?Object|?ClientRect} The intersection rect or undefined if no
- *     intersection is found.
- */
-function computeRectIntersection(rect1, rect2) {
-  var top = Math.max(rect1.top, rect2.top);
-  var bottom = Math.min(rect1.bottom, rect2.bottom);
-  var left = Math.max(rect1.left, rect2.left);
-  var right = Math.min(rect1.right, rect2.right);
-  var width = right - left;
-  var height = bottom - top;
-
-  return (width >= 0 && height >= 0) && {
-    top: top,
-    bottom: bottom,
-    left: left,
-    right: right,
-    width: width,
-    height: height
-  } || null;
-}
-
-
-/**
- * Shims the native getBoundingClientRect for compatibility with older IE.
- * @param {Element} el The element whose bounding rect to get.
- * @return {DOMRect|ClientRect} The (possibly shimmed) rect of the element.
- */
-function getBoundingClientRect(el) {
-  var rect;
-
-  try {
-    rect = el.getBoundingClientRect();
-  } catch (err) {
-    // Ignore Windows 7 IE11 "Unspecified error"
-    // https://github.com/w3c/IntersectionObserver/pull/205
-  }
-
-  if (!rect) return getEmptyRect();
-
-  // Older IE
-  if (!(rect.width && rect.height)) {
-    rect = {
-      top: rect.top,
-      right: rect.right,
-      bottom: rect.bottom,
-      left: rect.left,
-      width: rect.right - rect.left,
-      height: rect.bottom - rect.top
-    };
-  }
-  return rect;
-}
-
-
-/**
- * Returns an empty rect object. An empty rect is returned when an element
- * is not in the DOM.
- * @return {ClientRect} The empty rect.
- */
-function getEmptyRect() {
-  return {
-    top: 0,
-    bottom: 0,
-    left: 0,
-    right: 0,
-    width: 0,
-    height: 0
-  };
-}
-
-
-/**
- * Ensure that the result has all of the necessary fields of the DOMRect.
- * Specifically this ensures that `x` and `y` fields are set.
- *
- * @param {?DOMRect|?ClientRect} rect
- * @return {?DOMRect}
- */
-function ensureDOMRect(rect) {
-  // A `DOMRect` object has `x` and `y` fields.
-  if (!rect || 'x' in rect) {
-    return rect;
-  }
-  // A IE's `ClientRect` type does not have `x` and `y`. The same is the case
-  // for internally calculated Rect objects. For the purposes of
-  // `IntersectionObserver`, it's sufficient to simply mirror `left` and `top`
-  // for these fields.
-  return {
-    top: rect.top,
-    y: rect.top,
-    bottom: rect.bottom,
-    left: rect.left,
-    x: rect.left,
-    right: rect.right,
-    width: rect.width,
-    height: rect.height
-  };
-}
-
-
-/**
- * Inverts the intersection and bounding rect from the parent (frame) BCR to
- * the local BCR space.
- * @param {DOMRect|ClientRect} parentBoundingRect The parent's bound client rect.
- * @param {DOMRect|ClientRect} parentIntersectionRect The parent's own intersection rect.
- * @return {ClientRect} The local root bounding rect for the parent's children.
- */
-function convertFromParentRect(parentBoundingRect, parentIntersectionRect) {
-  var top = parentIntersectionRect.top - parentBoundingRect.top;
-  var left = parentIntersectionRect.left - parentBoundingRect.left;
-  return {
-    top: top,
-    left: left,
-    height: parentIntersectionRect.height,
-    width: parentIntersectionRect.width,
-    bottom: top + parentIntersectionRect.height,
-    right: left + parentIntersectionRect.width
-  };
-}
-
-
-/**
- * Checks to see if a parent element contains a child element (including inside
- * shadow DOM).
- * @param {Node} parent The parent element.
- * @param {Node} child The child element.
- * @return {boolean} True if the parent node contains the child node.
- */
-function containsDeep(parent, child) {
-  var node = child;
-  while (node) {
-    if (node == parent) return true;
-
-    node = getParentNode(node);
-  }
-  return false;
-}
-
-
-/**
- * Gets the parent node of an element or its host element if the parent node
- * is a shadow root.
- * @param {Node} node The node whose parent to get.
- * @return {Node|null} The parent node or null if no parent exists.
- */
-function getParentNode(node) {
-  var parent = node.parentNode;
-
-  if (node.nodeType == /* DOCUMENT */ 9 && node != document) {
-    // If this node is a document node, look for the embedding frame.
-    return getFrameElement(node);
-  }
-
-  // If the parent has element that is assigned through shadow root slot
-  if (parent && parent.assignedSlot) {
-    parent = parent.assignedSlot.parentNode
-  }
-
-  if (parent && parent.nodeType == 11 && parent.host) {
-    // If the parent is a shadow root, return the host element.
-    return parent.host;
-  }
-
-  return parent;
-}
-
-/**
- * Returns true if `node` is a Document.
- * @param {!Node} node
- * @returns {boolean}
- */
-function isDoc(node) {
-  return node && node.nodeType === 9;
-}
-
-
-// Exposes the constructors globally.
-window.IntersectionObserver = IntersectionObserver;
-window.IntersectionObserverEntry = IntersectionObserverEntry;
-
-}());