diff --git a/.changeset/silly-fishes-deny.md b/.changeset/silly-fishes-deny.md
new file mode 100644
index 000000000000..a77098c5978b
--- /dev/null
+++ b/.changeset/silly-fishes-deny.md
@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: allow custom element styles to be updated in HMR mode
diff --git a/.changeset/wild-mice-look.md b/.changeset/wild-mice-look.md
new file mode 100644
index 000000000000..535ace2fa53d
--- /dev/null
+++ b/.changeset/wild-mice-look.md
@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: inject styles correctly when mounting inside an iframe
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index 690cdb43454f..45e48e565e73 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -494,18 +494,7 @@ export function client_component(analysis, options) {
 
 		if (analysis.css.hash) {
 			// remove existing `<style>` element, in case CSS changed
-			accept_fn_body.unshift(
-				b.stmt(
-					b.call(
-						b.member(
-							b.call('document.querySelector', b.literal('#' + analysis.css.hash)),
-							'remove',
-							false,
-							true
-						)
-					)
-				)
-			);
+			accept_fn_body.unshift(b.stmt(b.call('$.cleanup_styles', b.literal(analysis.css.hash))));
 		}
 
 		const hmr = b.block([
diff --git a/packages/svelte/src/internal/client/dev/css.js b/packages/svelte/src/internal/client/dev/css.js
new file mode 100644
index 000000000000..96f67f7d1040
--- /dev/null
+++ b/packages/svelte/src/internal/client/dev/css.js
@@ -0,0 +1,31 @@
+/** @type {Map<String, Set<HTMLStyleElement>>} */
+var all_styles = new Map();
+
+/**
+ * @param {String} hash
+ * @param {HTMLStyleElement} style
+ */
+export function register_style(hash, style) {
+	var styles = all_styles.get(hash);
+
+	if (!styles) {
+		styles = new Set();
+		all_styles.set(hash, styles);
+	}
+
+	styles.add(style);
+}
+
+/**
+ * @param {String} hash
+ */
+export function cleanup_styles(hash) {
+	var styles = all_styles.get(hash);
+	if (!styles) return;
+
+	for (const style of styles) {
+		style.remove();
+	}
+
+	all_styles.delete(hash);
+}
diff --git a/packages/svelte/src/internal/client/dom/css.js b/packages/svelte/src/internal/client/dom/css.js
index dbf862543f1f..2c7efe899485 100644
--- a/packages/svelte/src/internal/client/dom/css.js
+++ b/packages/svelte/src/internal/client/dom/css.js
@@ -1,16 +1,22 @@
 import { DEV } from 'esm-env';
 import { queue_micro_task } from './task.js';
+import { register_style } from '../dev/css.js';
 
-var seen = new Set();
+var roots = new WeakMap();
 
 /**
  * @param {Node} anchor
  * @param {{ hash: string, code: string }} css
  * @param {boolean} [is_custom_element]
  */
-export function append_styles(anchor, css, is_custom_element = false) {
+export function append_styles(anchor, css, is_custom_element) {
 	// in dev, always check the DOM, so that styles can be replaced with HMR
 	if (!DEV && !is_custom_element) {
+		var doc = /** @type {Document} */ (anchor.ownerDocument);
+
+		if (!roots.has(doc)) roots.set(doc, new Set());
+		const seen = roots.get(doc);
+
 		if (seen.has(css)) return;
 		seen.add(css);
 	}
@@ -29,6 +35,10 @@ export function append_styles(anchor, css, is_custom_element = false) {
 			style.textContent = css.code;
 
 			target.appendChild(style);
+
+			if (DEV) {
+				register_style(css.hash, style);
+			}
 		}
 	});
 }
diff --git a/packages/svelte/src/internal/client/index.js b/packages/svelte/src/internal/client/index.js
index 519a41248685..ead3bfa3653d 100644
--- a/packages/svelte/src/internal/client/index.js
+++ b/packages/svelte/src/internal/client/index.js
@@ -1,4 +1,5 @@
 export { FILENAME, HMR } from '../../constants.js';
+export { cleanup_styles } from './dev/css.js';
 export { add_locations } from './dev/elements.js';
 export { hmr } from './dev/hmr.js';
 export {
diff --git a/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/App.svelte b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/App.svelte
new file mode 100644
index 000000000000..8c44e7951ead
--- /dev/null
+++ b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/App.svelte
@@ -0,0 +1,19 @@
+<script>
+	import { mount } from 'svelte';
+	import Child from './Child.svelte';
+
+	let { count } = $props();
+
+	let iframe;
+
+	$effect(() => {
+		mount(Child, {
+			target: iframe.contentWindow.document.body,
+			props: {
+				count
+			}
+		});
+	});
+</script>
+
+<iframe title="container" bind:this={iframe}></iframe>
diff --git a/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/Child.svelte b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/Child.svelte
new file mode 100644
index 000000000000..dac93751b8af
--- /dev/null
+++ b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/Child.svelte
@@ -0,0 +1,13 @@
+<svelte:options css="injected" />
+
+<script>
+	let { count } = $props();
+</script>
+
+<h1>count: {count}</h1>
+
+<style>
+	h1 {
+		color: red;
+	}
+</style>
diff --git a/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/_config.js b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/_config.js
new file mode 100644
index 000000000000..0f04e9659a3f
--- /dev/null
+++ b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/_config.js
@@ -0,0 +1,22 @@
+import { flushSync } from 'svelte';
+import { test } from '../../assert';
+
+export default test({
+	async test({ target, assert }) {
+		const button = target.querySelector('button');
+		const h1 = () =>
+			/** @type {HTMLHeadingElement} */ (
+				/** @type {Window} */ (
+					target.querySelector('iframe')?.contentWindow
+				).document.querySelector('h1')
+			);
+
+		assert.equal(h1().textContent, 'count: 0');
+		assert.equal(getComputedStyle(h1()).color, 'rgb(255, 0, 0)');
+
+		flushSync(() => button?.click());
+
+		assert.equal(h1().textContent, 'count: 1');
+		assert.equal(getComputedStyle(h1()).color, 'rgb(255, 0, 0)');
+	}
+});
diff --git a/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/main.svelte b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/main.svelte
new file mode 100644
index 000000000000..358698f8ecdd
--- /dev/null
+++ b/packages/svelte/tests/runtime-browser/samples/mount-in-iframe/main.svelte
@@ -0,0 +1,11 @@
+<script>
+	import App from './App.svelte';
+
+	let count = $state(0);
+</script>
+
+<button onclick={() => (count += 1)}>remount</button>
+
+{#key count}
+	<App {count} />
+{/key}