feat(runtime-browser): add support for <svelte:element this="contents"> to render children without wrapper

- Added handling in element block to treat "contents" as no-wrapper rendering
- Added runtime-browser test for this behavior

Author: dukhevych

.changeset/curvy-experts-join.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+Add support for `<svelte:element this="contents">` to render children without wrapper element

packages/svelte/src/internal/client/dom/blocks/svelte-element.js

@@ -96,7 +96,15 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 			}
 		}
 
-		if (next_tag && next_tag !== current_tag) {
+		if (next_tag === 'contents') {
+			// Don't mount anything; just execute render_fn directly with anchor
+			effect = branch(() => {
+				if (render_fn) {
+					render_fn(anchor.parentNode, anchor);
+				}
+				/** @type {Effect} */ (active_effect).nodes_end = anchor;
+			});
+		} else if (next_tag && next_tag !== current_tag) {
 			effect = branch(() => {
 				element = hydrating
 					? /** @type {Element} */ (element)

packages/svelte/tests/runtime-browser/samples/svelte-element-contents/_config.js

@@ -0,0 +1,12 @@
+import { test } from '../../assert';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, window }) {
+		await new Promise((r) => setTimeout(r, 1000));
+		assert.htmlEqual(
+			window.document.body.innerHTML,
+			`<main><strong>Hello</strong></main>`
+		);
+	}
+});

packages/svelte/tests/runtime-browser/samples/svelte-element-contents/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let tag = 'contents';
+</script>
+
+<svelte:element this={tag}>
+	<strong>Hello</strong>
+</svelte:element>