diff --git a/src/style.js b/src/style.js
index d7f1ef0975..e247afece4 100644
--- a/src/style.js
+++ b/src/style.js
@@ -414,8 +414,9 @@ export function impliedNumber(value, impliedValue) {
   if ((value = number(value)) !== impliedValue) return value;
 }
 
+// https://www.w3.org/TR/CSS21/grammar.html
 const validClassName =
-  /^-?([_a-z]|[\240-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])([_a-z0-9-]|[\240-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])*$/;
+  /^-?([_a-z]|[\240-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])([_a-z0-9-]|[\240-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])*$/i;
 
 export function maybeClassName(name) {
   if (name === undefined) return `plot-${Math.random().toString(16).slice(2)}`;
diff --git a/test/style-test.js b/test/style-test.js
new file mode 100644
index 0000000000..de55220f69
--- /dev/null
+++ b/test/style-test.js
@@ -0,0 +1,47 @@
+import assert from "assert";
+import {maybeClassName} from "../src/style.js";
+
+it("maybeClassName allows typical class names", () => {
+  assert.strictEqual(maybeClassName("foo"), "foo");
+  assert.strictEqual(maybeClassName("foo2"), "foo2");
+  assert.strictEqual(maybeClassName("foo-bar"), "foo-bar");
+  assert.strictEqual(maybeClassName("-bar"), "-bar");
+});
+
+it("maybeClassName allows escaped class names", () => {
+  assert.strictEqual(maybeClassName("\\1234"), "\\1234");
+  assert.strictEqual(maybeClassName("\\64"), "\\64");
+  assert.strictEqual(maybeClassName("\\ "), "\\ ");
+});
+
+it("maybeClassName allows mixed-case class names", () => {
+  assert.strictEqual(maybeClassName("FOO"), "FOO");
+  assert.strictEqual(maybeClassName("fooBar"), "fooBar");
+});
+
+it("maybeClassName disallows invalid class names", () => {
+  assert.throws(() => maybeClassName(" "), /invalid class name/);
+  assert.throws(() => maybeClassName("42"), /invalid class name/);
+  assert.throws(() => maybeClassName("foo "), /invalid class name/);
+  assert.throws(() => maybeClassName(" foo"), /invalid class name/);
+  assert.throws(() => maybeClassName(".foo"), /invalid class name/);
+  assert.throws(() => maybeClassName("[foo]"), /invalid class name/);
+  assert.throws(() => maybeClassName("💩"), /invalid class name/);
+  assert.throws(() => maybeClassName("--hi"), /invalid class name/);
+  assert.throws(() => maybeClassName("\\"), /invalid class name/);
+});
+
+it("maybeClassName coerces to strings", () => {
+  assert.strictEqual(maybeClassName(["foo"]), "foo");
+  assert.strictEqual(maybeClassName({toString: () => "foo-bar"}), "foo-bar");
+});
+
+it("maybeClassName generates distinct random class names", () => {
+  const names = new Set();
+  for (let i = 0; i < 100; ++i) {
+    const name = maybeClassName();
+    assert.match(name, /^plot-[0-9a-f]{6,}$/);
+    assert.strictEqual(names.has(name), false);
+    names.add(name);
+  }
+});