From c9ebd6a88553a3041d68c6063ec095fdd601f4b7 Mon Sep 17 00:00:00 2001
From: Elliott Johnson <sejohnson@torchcloudconsulting.com>
Date: Wed, 21 Jan 2026 14:09:30 -0700
Subject: [PATCH] feat: export `parseCss` from `svelte/compiler` (#17496)

* feat: export  from

* changeset

* fix: Errors

* set_source, so state is correct if an error occurs

* simplify

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/loud-buses-guess.md                |   5 +
 packages/svelte/src/compiler/index.js         |  26 +++-
 .../src/compiler/phases/1-parse/index.js      |  14 ++
 .../src/compiler/phases/1-parse/read/style.js |  30 ++--
 packages/svelte/tests/css-parse.test.ts       | 138 ++++++++++++++++++
 packages/svelte/types/index.d.ts              |   6 +
 6 files changed, 205 insertions(+), 14 deletions(-)
 create mode 100644 .changeset/loud-buses-guess.md
 create mode 100644 packages/svelte/tests/css-parse.test.ts

diff --git a/.changeset/loud-buses-guess.md b/.changeset/loud-buses-guess.md
new file mode 100644
index 0000000000..d6c7c3d5a4
--- /dev/null
+++ b/.changeset/loud-buses-guess.md
@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: export `parseCss` from `svelte/compiler`
diff --git a/packages/svelte/src/compiler/index.js b/packages/svelte/src/compiler/index.js
index 9095a2a2f2..acfef6a320 100644
--- a/packages/svelte/src/compiler/index.js
+++ b/packages/svelte/src/compiler/index.js
@@ -3,8 +3,9 @@
 /** @import { AST } from './public.js' */
 import { walk as zimmerframe_walk } from 'zimmerframe';
 import { convert } from './legacy.js';
-import { parse as _parse } from './phases/1-parse/index.js';
+import { parse as _parse, Parser } from './phases/1-parse/index.js';
 import { remove_typescript_nodes } from './phases/1-parse/remove_typescript_nodes.js';
+import { parse_stylesheet } from './phases/1-parse/read/style.js';
 import { analyze_component, analyze_module } from './phases/2-analyze/index.js';
 import { transform_component, transform_module } from './phases/3-transform/index.js';
 import { validate_component_options, validate_module_options } from './validate-options.js';
@@ -118,6 +119,29 @@ export function parse(source, { modern, loose } = {}) {
 	return to_public_ast(source, ast, modern);
 }
 
+/**
+ * The parseCss function parses a CSS stylesheet, returning its abstract syntax tree.
+ *
+ * @param {string} source The CSS source code
+ * @returns {Omit<AST.CSS.StyleSheet, 'attributes' | 'content'>}
+ */
+export function parseCss(source) {
+	source = remove_bom(source);
+	state.reset({ warning: () => false, filename: undefined });
+
+	state.set_source(source);
+
+	const parser = Parser.forCss(source);
+	const children = parse_stylesheet(parser);
+
+	return {
+		type: 'StyleSheet',
+		start: 0,
+		end: source.length,
+		children
+	};
+}
+
 /**
  * @param {string} source
  * @param {AST.Root} ast
diff --git a/packages/svelte/src/compiler/phases/1-parse/index.js b/packages/svelte/src/compiler/phases/1-parse/index.js
index 3b5f83d58f..88b4352e8a 100644
--- a/packages/svelte/src/compiler/phases/1-parse/index.js
+++ b/packages/svelte/src/compiler/phases/1-parse/index.js
@@ -34,6 +34,20 @@ export class Parser {
 	/** */
 	index = 0;
 
+	/**
+	 * Creates a minimal parser instance for CSS-only parsing.
+	 * Skips Svelte component parsing setup.
+	 * @param {string} source
+	 * @returns {Parser}
+	 */
+	static forCss(source) {
+		const parser = Object.create(Parser.prototype);
+		parser.template = source;
+		parser.index = 0;
+		parser.loose = false;
+		return parser;
+	}
+
 	/** Whether we're parsing in TypeScript mode */
 	ts = false;
 
diff --git a/packages/svelte/src/compiler/phases/1-parse/read/style.js b/packages/svelte/src/compiler/phases/1-parse/read/style.js
index 80ab234d92..c88a3707c1 100644
--- a/packages/svelte/src/compiler/phases/1-parse/read/style.js
+++ b/packages/svelte/src/compiler/phases/1-parse/read/style.js
@@ -24,10 +24,11 @@ const REGEX_HTML_COMMENT_CLOSE = /-->/;
  */
 export default function read_style(parser, start, attributes) {
 	const content_start = parser.index;
-	const children = read_body(parser, '</style');
+	const children = read_body(parser, (p) => p.match('</style') || p.index >= p.template.length);
 	const content_end = parser.index;
 
-	parser.read(/^<\/style\s*>/);
+	parser.eat('</style', true);
+	parser.read(/^\s*>/);
 
 	return {
 		type: 'StyleSheet',
@@ -46,20 +47,14 @@ export default function read_style(parser, start, attributes) {
 
 /**
  * @param {Parser} parser
- * @param {string} close
- * @returns {any[]}
+ * @param {(parser: Parser) => boolean} finished
+ * @returns {Array<AST.CSS.Rule | AST.CSS.Atrule>}
  */
-function read_body(parser, close) {
+function read_body(parser, finished) {
 	/** @type {Array<AST.CSS.Rule | AST.CSS.Atrule>} */
 	const children = [];
 
-	while (parser.index < parser.template.length) {
-		allow_comment_or_whitespace(parser);
-
-		if (parser.match(close)) {
-			return children;
-		}
-
+	while ((allow_comment_or_whitespace(parser), !finished(parser))) {
 		if (parser.match('@')) {
 			children.push(read_at_rule(parser));
 		} else {
@@ -67,7 +62,7 @@ function read_body(parser, close) {
 		}
 	}
 
-	e.expected_token(parser.template.length, close);
+	return children;
 }
 
 /**
@@ -627,3 +622,12 @@ function allow_comment_or_whitespace(parser) {
 		parser.allow_whitespace();
 	}
 }
+
+/**
+ * Parse standalone CSS content (not wrapped in `<style>`).
+ * @param {Parser} parser
+ * @returns {Array<AST.CSS.Rule | AST.CSS.Atrule>}
+ */
+export function parse_stylesheet(parser) {
+	return read_body(parser, (p) => p.index >= p.template.length);
+}
diff --git a/packages/svelte/tests/css-parse.test.ts b/packages/svelte/tests/css-parse.test.ts
new file mode 100644
index 0000000000..fcb9bdc010
--- /dev/null
+++ b/packages/svelte/tests/css-parse.test.ts
@@ -0,0 +1,138 @@
+import { assert, describe, it } from 'vitest';
+import { parseCss } from 'svelte/compiler';
+
+describe('parseCss', () => {
+	it('parses a simple rule', () => {
+		const ast = parseCss('div { color: red; }');
+		assert.equal(ast.type, 'StyleSheet');
+		assert.equal(ast.children.length, 1);
+		assert.equal(ast.children[0].type, 'Rule');
+	});
+
+	it('parses at-rules', () => {
+		const ast = parseCss('@media (min-width: 800px) { div { color: red; } }');
+		assert.equal(ast.children.length, 1);
+		assert.equal(ast.children[0].type, 'Atrule');
+		if (ast.children[0].type === 'Atrule') {
+			assert.equal(ast.children[0].name, 'media');
+		}
+	});
+
+	it('parses @import', () => {
+		const ast = parseCss("@import 'foo.css';");
+		assert.equal(ast.children.length, 1);
+		assert.equal(ast.children[0].type, 'Atrule');
+		if (ast.children[0].type === 'Atrule') {
+			assert.equal(ast.children[0].name, 'import');
+			assert.equal(ast.children[0].block, null);
+		}
+	});
+
+	it('parses multiple rules', () => {
+		const ast = parseCss('div { color: red; } span { color: blue; }');
+		assert.equal(ast.children.length, 2);
+	});
+
+	it('has correct start/end positions', () => {
+		const ast = parseCss('div { color: red; }');
+		assert.equal(ast.start, 0);
+		assert.equal(ast.end, 19);
+	});
+
+	it('strips BOM', () => {
+		const ast = parseCss('\uFEFFdiv { color: red; }');
+		assert.equal(ast.start, 0);
+		assert.equal(ast.end, 19);
+	});
+
+	it('parses nested rules', () => {
+		const ast = parseCss('div { color: red; span { color: blue; } }');
+		assert.equal(ast.children.length, 1);
+		const rule = ast.children[0];
+		assert.equal(rule.type, 'Rule');
+		if (rule.type === 'Rule') {
+			assert.equal(rule.block.children.length, 2); // declaration + nested rule
+		}
+	});
+
+	it('parses empty stylesheet', () => {
+		const ast = parseCss('');
+		assert.equal(ast.type, 'StyleSheet');
+		assert.equal(ast.children.length, 0);
+		assert.equal(ast.start, 0);
+		assert.equal(ast.end, 0);
+	});
+
+	it('parses whitespace-only stylesheet', () => {
+		const ast = parseCss('   \n\t  ');
+		assert.equal(ast.children.length, 0);
+	});
+
+	it('parses comments', () => {
+		const ast = parseCss('/* comment */ div { color: red; }');
+		assert.equal(ast.children.length, 1);
+		assert.equal(ast.children[0].type, 'Rule');
+	});
+
+	it('parses complex selectors', () => {
+		const ast = parseCss('div > span + p ~ a { color: red; }');
+		assert.equal(ast.children.length, 1);
+		const rule = ast.children[0];
+		if (rule.type === 'Rule') {
+			assert.equal(rule.prelude.type, 'SelectorList');
+			assert.equal(rule.prelude.children.length, 1);
+			// div > span + p ~ a has 4 relative selectors
+			assert.equal(rule.prelude.children[0].children.length, 4);
+		}
+	});
+
+	it('parses pseudo-classes and pseudo-elements', () => {
+		const ast = parseCss('div:hover::before { color: red; }');
+		assert.equal(ast.children.length, 1);
+		const rule = ast.children[0];
+		if (rule.type === 'Rule') {
+			const selectors = rule.prelude.children[0].children[0].selectors;
+			assert.equal(selectors.length, 3); // div, :hover, ::before
+			assert.equal(selectors[0].type, 'TypeSelector');
+			assert.equal(selectors[1].type, 'PseudoClassSelector');
+			assert.equal(selectors[2].type, 'PseudoElementSelector');
+		}
+	});
+
+	it('parses @keyframes', () => {
+		const ast = parseCss('@keyframes fade { from { opacity: 0; } to { opacity: 1; } }');
+		assert.equal(ast.children.length, 1);
+		assert.equal(ast.children[0].type, 'Atrule');
+		if (ast.children[0].type === 'Atrule') {
+			assert.equal(ast.children[0].name, 'keyframes');
+			assert.notEqual(ast.children[0].block, null);
+		}
+	});
+
+	it('parses class and id selectors', () => {
+		const ast = parseCss('.foo#bar { color: red; }');
+		assert.equal(ast.children.length, 1);
+		const rule = ast.children[0];
+		if (rule.type === 'Rule') {
+			const selectors = rule.prelude.children[0].children[0].selectors;
+			assert.equal(selectors.length, 2);
+			assert.equal(selectors[0].type, 'ClassSelector');
+			assert.equal(selectors[1].type, 'IdSelector');
+		}
+	});
+
+	it('parses attribute selectors', () => {
+		const ast = parseCss('[data-foo="bar"] { color: red; }');
+		assert.equal(ast.children.length, 1);
+		const rule = ast.children[0];
+		if (rule.type === 'Rule') {
+			const selectors = rule.prelude.children[0].children[0].selectors;
+			assert.equal(selectors.length, 1);
+			assert.equal(selectors[0].type, 'AttributeSelector');
+			if (selectors[0].type === 'AttributeSelector') {
+				assert.equal(selectors[0].name, 'data-foo');
+				assert.equal(selectors[0].value, 'bar');
+			}
+		}
+	});
+});
diff --git a/packages/svelte/types/index.d.ts b/packages/svelte/types/index.d.ts
index c0b89d8736..9e569126bb 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -884,6 +884,12 @@ declare module 'svelte/compiler' {
 		modern?: false;
 		loose?: boolean;
 	} | undefined): Record<string, any>;
+	/**
+	 * The parseCss function parses a CSS stylesheet, returning its abstract syntax tree.
+	 *
+	 * @param source The CSS source code
+	 * */
+	export function parseCss(source: string): Omit<AST.CSS.StyleSheet, "attributes" | "content">;
 	/**
 	 * @deprecated Replace this with `import { walk } from 'estree-walker'`
 	 * */