diff --git a/packages/svelte/src/compiler/phases/2-analyze/css/css-warn.js b/packages/svelte/src/compiler/phases/2-analyze/css/css-warn.js
new file mode 100644
index 0000000000..32dfab32ab
--- /dev/null
+++ b/packages/svelte/src/compiler/phases/2-analyze/css/css-warn.js
@@ -0,0 +1,30 @@
+import { walk } from 'zimmerframe';
+import { warn } from '../../../warnings.js';
+
+/**
+ * @param {import('#compiler').Css.StyleSheet} stylesheet
+ * @param {import('../../types.js').RawWarning[]} warnings
+ */
+export function warn_unused(stylesheet, warnings) {
+	walk(stylesheet, { warnings, stylesheet }, visitors);
+}
+
+/** @type {import('zimmerframe').Visitors<import('#compiler').Css.Node, { warnings: import('../../types.js').RawWarning[], stylesheet: import('#compiler').Css.StyleSheet }>} */
+const visitors = {
+	ComplexSelector(node, context) {
+		if (!node.metadata.used) {
+			for (let i = context.path.length - 1; i >= 0; i--) {
+				const parent = context.path[i];
+				if (parent.type === 'RelativeSelector' && parent.metadata.is_global) {
+					return; // no need to recurse; everything below is global, too
+				}
+			}
+
+			const content = context.state.stylesheet.content;
+			const text = content.styles.substring(node.start - content.start, node.end - content.start);
+			warn(context.state.warnings, node, context.path, 'css-unused-selector', text);
+		}
+
+		context.next();
+	}
+};
diff --git a/packages/svelte/src/compiler/phases/2-analyze/index.js b/packages/svelte/src/compiler/phases/2-analyze/index.js
index 887c961734..d9c6dfe64a 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/index.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/index.js
@@ -23,6 +23,7 @@ import { should_proxy_or_freeze } from '../3-transform/client/utils.js';
 import { analyze_css } from './css/css-analyze.js';
 import { prune } from './css/css-prune.js';
 import { hash } from './utils.js';
+import { warn_unused } from './css/css-warn.js';
 
 /**
  * @param {import('#compiler').Script | null} script
@@ -548,6 +549,7 @@ export function analyze_component(root, source, options) {
 		for (const element of analysis.elements) {
 			prune(analysis.css.ast, element);
 		}
+		warn_unused(analysis.css.ast, analysis.warnings);
 
 		outer: for (const element of analysis.elements) {
 			if (element.metadata.scoped) {
diff --git a/packages/svelte/src/compiler/warnings.js b/packages/svelte/src/compiler/warnings.js
index 7b863ef2b1..98dc07d7a0 100644
--- a/packages/svelte/src/compiler/warnings.js
+++ b/packages/svelte/src/compiler/warnings.js
@@ -7,7 +7,8 @@ import {
 
 /** @satisfies {Warnings} */
 const css = {
-	'unused-selector': () => 'Unused CSS selector'
+	/** @param {string} name */
+	'css-unused-selector': (name) => `Unused CSS selector "${name}"`
 };
 
 /** @satisfies {Warnings} */
diff --git a/packages/svelte/tests/css/samples/attribute-selector-case-sensitive/_config.js b/packages/svelte/tests/css/samples/attribute-selector-case-sensitive/_config.js
new file mode 100644
index 0000000000..4be32cf365
--- /dev/null
+++ b/packages/svelte/tests/css/samples/attribute-selector-case-sensitive/_config.js
@@ -0,0 +1,20 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		{
+			code: 'css-unused-selector',
+			end: {
+				character: 44,
+				column: 14,
+				line: 4
+			},
+			message: 'Unused CSS selector "p[type=\'B\' s]"',
+			start: {
+				character: 31,
+				column: 1,
+				line: 4
+			}
+		}
+	]
+});
diff --git a/packages/svelte/tests/css/samples/descendant-selector-unmatched/_config.js b/packages/svelte/tests/css/samples/descendant-selector-unmatched/_config.js
new file mode 100644
index 0000000000..509e5faec9
--- /dev/null
+++ b/packages/svelte/tests/css/samples/descendant-selector-unmatched/_config.js
@@ -0,0 +1,20 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		{
+			code: 'css-unused-selector',
+			end: {
+				character: 33,
+				column: 6,
+				line: 6
+			},
+			message: 'Unused CSS selector "x y z"',
+			start: {
+				character: 28,
+				column: 1,
+				line: 6
+			}
+		}
+	]
+});
diff --git a/packages/svelte/tests/css/samples/general-siblings-combinator-rendertag-global/_config.js b/packages/svelte/tests/css/samples/general-siblings-combinator-rendertag-global/_config.js
index 2143e5e575..2038788f67 100644
--- a/packages/svelte/tests/css/samples/general-siblings-combinator-rendertag-global/_config.js
+++ b/packages/svelte/tests/css/samples/general-siblings-combinator-rendertag-global/_config.js
@@ -2,12 +2,19 @@ import { test } from '../../test';
 
 export default test({
 	warnings: [
-		// TODO
-		// {
-		// 	code: 'css-unused-selector',
-		// 	message: 'Unused CSS selector ".a ~ .b"',
-		// 	start: { character: 111, column: 1, line: 10 },
-		// 	end: { character: 118, column: 8, line: 10 }
-		// },
+		{
+			code: 'css-unused-selector',
+			end: {
+				character: 479,
+				column: 19,
+				line: 22
+			},
+			message: 'Unused CSS selector ":global(.x) + .bar"',
+			start: {
+				character: 461,
+				column: 1,
+				line: 22
+			}
+		}
 	]
 });
diff --git a/packages/svelte/tests/css/samples/general-siblings-combinator-slot-global/_config.js b/packages/svelte/tests/css/samples/general-siblings-combinator-slot-global/_config.js
index 2143e5e575..2038788f67 100644
--- a/packages/svelte/tests/css/samples/general-siblings-combinator-slot-global/_config.js
+++ b/packages/svelte/tests/css/samples/general-siblings-combinator-slot-global/_config.js
@@ -2,12 +2,19 @@ import { test } from '../../test';
 
 export default test({
 	warnings: [
-		// TODO
-		// {
-		// 	code: 'css-unused-selector',
-		// 	message: 'Unused CSS selector ".a ~ .b"',
-		// 	start: { character: 111, column: 1, line: 10 },
-		// 	end: { character: 118, column: 8, line: 10 }
-		// },
+		{
+			code: 'css-unused-selector',
+			end: {
+				character: 479,
+				column: 19,
+				line: 22
+			},
+			message: 'Unused CSS selector ":global(.x) + .bar"',
+			start: {
+				character: 461,
+				column: 1,
+				line: 22
+			}
+		}
 	]
 });
diff --git a/packages/svelte/tests/css/test.ts b/packages/svelte/tests/css/test.ts
index 71b8e9644c..d17f7e9c6b 100644
--- a/packages/svelte/tests/css/test.ts
+++ b/packages/svelte/tests/css/test.ts
@@ -8,12 +8,17 @@ import { mount, unmount } from 'svelte';
 import { suite, type BaseTest } from '../suite.js';
 import type { CompileOptions, Warning } from '#compiler';
 
-// function normalize_warning(warning) {
-// 	warning.frame = warning.frame.replace(/^\n/, '').replace(/^\t+/gm, '').replace(/\s+$/gm, '');
-// 	delete warning.filename;
-// 	delete warning.toString;
-// 	return warning;
-// }
+function normalize_warning(warning: Warning) {
+	delete warning.filename;
+	return warning;
+}
+
+function load_warnings(path: string) {
+	if (!fs.existsSync(path)) {
+		return [];
+	}
+	return JSON.parse(fs.readFileSync(path, 'utf-8')).map(normalize_warning);
+}
 
 interface CssTest extends BaseTest {
 	compileOptions?: Partial<CompileOptions>;
@@ -22,9 +27,6 @@ interface CssTest extends BaseTest {
 }
 
 const { test, run } = suite<CssTest>(async (config, cwd) => {
-	// TODO
-	// const expected_warnings = (config.warnings || []).map(normalize_warning);
-
 	await compile_directory(cwd, 'client', { cssHash: () => 'svelte-xyz', ...config.compileOptions });
 	await compile_directory(cwd, 'server', { cssHash: () => 'svelte-xyz', ...config.compileOptions });
 
@@ -33,11 +35,11 @@ const { test, run } = suite<CssTest>(async (config, cwd) => {
 
 	assert.equal(dom_css, ssr_css);
 
-	// TODO reenable
-	// const dom_warnings = dom.warnings.map(normalize_warning);
-	// const ssr_warnings = ssr.warnings.map(normalize_warning);
-	// assert.deepEqual(dom_warnings, ssr_warnings);
-	// assert.deepEqual(dom_warnings.map(normalize_warning), expected_warnings);
+	const dom_warnings = load_warnings(`${cwd}/_output/client/input.svelte.warnings.json`);
+	const ssr_warnings = load_warnings(`${cwd}/_output/server/input.svelte.warnings.json`);
+	const expected_warnings = (config.warnings || []).map(normalize_warning);
+	assert.deepEqual(dom_warnings, ssr_warnings);
+	assert.deepEqual(dom_warnings.map(normalize_warning), expected_warnings);
 
 	const expected = {
 		html: try_read_file(`${cwd}/expected.html`),
diff --git a/packages/svelte/tests/helpers.js b/packages/svelte/tests/helpers.js
index 16c923391d..f640e519f9 100644
--- a/packages/svelte/tests/helpers.js
+++ b/packages/svelte/tests/helpers.js
@@ -71,7 +71,7 @@ export async function compile_directory(
 	for (const file of glob('**', { cwd, filesOnly: true })) {
 		if (file.startsWith('_')) continue;
 
-		let text = fs.readFileSync(`${cwd}/${file}`, 'utf-8');
+		let text = fs.readFileSync(`${cwd}/${file}`, 'utf-8').replace(/\r\n/g, '\n');
 		let opts = {
 			filename: path.join(cwd, file),
 			...compileOptions,
@@ -138,6 +138,10 @@ export async function compile_directory(
 					write(`${output_dir}/${file}.css.map`, JSON.stringify(compiled.css.map, null, '\t'));
 				}
 			}
+
+			if (compiled.warnings.length > 0) {
+				write(`${output_dir}/${file}.warnings.json`, JSON.stringify(compiled.warnings, null, '\t'));
+			}
 		}
 	}
 }