fix: don't apply scoped CSS class to elements inside <svelte:head> (#18160)

skip elements inside head, they should not get hashes

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/hungry-plums-roll.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: never apply class hash to elements inside `<svelte:head>`

packages/svelte/src/compiler/phases/2-analyze/css/css-prune.js

@@ -122,6 +122,13 @@ const any_selector = {
  */
 const seen = new Set();
 
+/**
+ * @param {Compiler.AST.RegularElement | Compiler.AST.SvelteElement | Compiler.AST.RenderTag | Compiler.AST.Component | Compiler.AST.SvelteComponent | Compiler.AST.SvelteSelf} node
+ */
+function is_inside_svelte_head(node) {
+	return node.metadata.path.some((ancestor) => ancestor.type === 'SvelteHead');
+}
+
 /**
  *
  * @param {Compiler.AST.CSS.StyleSheet} stylesheet
@@ -143,6 +150,9 @@ export function prune(stylesheet, elements) {
 				seen.clear();
 
 				if (
+					// Elements rendered through <svelte:head> are not style-scopable.
+					// Prevent css hash injection (class="s-...") on tags like <meta>, <link>, <script>.
+					!is_inside_svelte_head(element) &&
 					apply_selector(
 						selectors,
 						/** @type {Compiler.AST.CSS.Rule} */ (node.metadata.rule),

packages/svelte/tests/hydration/samples/head-css-no-class/_expected_head.html

@@ -0,0 +1 @@
+<meta name="author" content="Re:Designed"> <link rel="author" href="https://example.com"> <script type="application/ld+json"></script>

packages/svelte/tests/hydration/samples/head-css-no-class/main.svelte

@@ -0,0 +1,11 @@
+<svelte:head>
+	<meta name="author" content="Re:Designed" />
+	<link rel="author" href="https://example.com" />
+	<script type="application/ld+json"></script>
+</svelte:head>
+
+<div>dummy</div>
+
+<style>
+	* { color: red }
+</style>