The exception are `:not` selectors with descendant selectors, as that means "look up the tree" and we need to scope all ancestor elements in that case.

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

@@ -162,9 +162,7 @@ const visitors = {
 };
 
 /**
- * Discard trailing `:global(...)` selectors without a `:has/is/where(...)` modifier, these are unused for scoping purposes
+ * Discard trailing `:global(...)` selectors without a `:has/is/where/not(...)` modifier, these are unused for scoping purposes
- * `:not(...)` modifiers are not considered, because they should stay unscoped, because scoping them would achieve the
- * opposite of what we want, because they are then _more_ likely to bleed out of the component.
  * @param {Compiler.Css.ComplexSelector} node
  */
 function truncate(node) {
@@ -174,13 +172,16 @@ function truncate(node) {
 			// not after a :global selector
 			!metadata.is_global_like &&
 			!(first.type === 'PseudoClassSelector' && first.name === 'global' && first.args === null) &&
-			// not a :global(...) without a :has/is/where(...) modifier
+			// not a :global(...) without a :has/is/where/not(...) modifier
 			(!metadata.is_global ||
 				selectors.some(
 					(selector) =>
 						selector.type === 'PseudoClassSelector' &&
 						selector.args !== null &&
-						(selector.name === 'has' || selector.name === 'is' || selector.name === 'where')
+						(selector.name === 'has' ||
+							selector.name === 'is' ||
+							selector.name === 'where' ||
+							selector.name === 'not')
 				))
 		);
 	});
@@ -511,9 +512,34 @@ function relative_selector_might_apply_to_node(relative_selector, rule, element,
 				// We came across a :global, everything beyond it is global and therefore a potential match
 				if (name === 'global' && selector.args === null) return true;
 
-				// We ignore :not(...) because its contents should stay unscoped. Scoping them would achieve the
+				// :not(...) contents should stay unscoped. Scoping them would achieve the opposite of what we want,
-				// opposite of what we want, because they are then _more_ likely to bleed out of the component,
+				// because they are then _more_ likely to bleed out of the component. The exception is complex selectors
-				// because there would be more chances of the inner selector not matching, which means `:not` matches.
+				// with descendants, in which case we scope them all.
+				if (name === 'not' && selector.args) {
+					for (const complex_selector of selector.args.children) {
+						complex_selector.metadata.used = true;
+						const relative = truncate(complex_selector);
+
+						if (complex_selector.children.length > 1) {
+							// foo:not(bar foo) means that bar is an ancestor of foo (side note: ending with foo is the only way the selector make sense).
+							// We can't fully check if that actually matches with our current algorithm, so we just assume it does.
+							// The result may not match a real element, so the only drawback is the missing prune.
+							for (const selector of relative) {
+								selector.metadata.scoped = true;
+							}
+
+							/** @type {Compiler.AST.RegularElement | Compiler.AST.SvelteElement | null} */
+							let el = element;
+							while (el) {
+								el.metadata.scoped = true;
+								el = get_element_parent(el);
+							}
+						}
+					}
+
+					break;
+				}
+
 				if ((name === 'is' || name === 'where') && selector.args) {
 					let matched = false;
 

packages/svelte/src/compiler/phases/3-transform/css/index.js

@@ -355,19 +355,9 @@ const visitors = {
 		context.state.specificity.bumped = before_bumped;
 	},
 	PseudoClassSelector(node, context) {
-		if (node.name === 'is' || node.name === 'where' || node.name === 'has') {
+		if (node.name === 'is' || node.name === 'where' || node.name === 'has' || node.name === 'not') {
 			context.next();
 		}
-		if (node.name === 'not' && node.args) {
-			for (const complex_selector of node.args.children) {
-				for (const relative_selector of complex_selector.children) {
-					if (relative_selector.metadata.is_global) {
-						const global = /** @type {Css.PseudoClassSelector} */ (relative_selector.selectors[0]);
-						remove_global_pseudo_class(global, relative_selector.combinator, context.state);
-					}
-				}
-			}
-		}
 	}
 };
 

packages/svelte/tests/css/samples/not-selector-global/expected.css

@@ -9,11 +9,21 @@
 		color: green;
 	}
 	.x .svelte-xyz:not(p) {
-		color: red; /* TODO would be nice to prune this one day */
+		color: green;
 	}
 	.x:not(p) {
-		color: red; /* TODO would be nice to prune this one day */
+		color: green;
 	}
 	.x .svelte-xyz:not(.unused) {
 		color: green;
 	}
+
+	span:not(p.svelte-xyz span:where(.svelte-xyz)) {
+		color: green;
+	}
+	span.svelte-xyz:not(p span) {
+		color: green;
+	}
+	span:not(p span) {
+		color: green;
+	}

packages/svelte/tests/css/samples/not-selector-global/expected.html

@@ -1 +1,3 @@
-<p class="foo svelte-xyz">foo</p> <p class="bar svelte-xyz">bar</p>
+<p class="foo svelte-xyz">foo</p>
+<p class="bar svelte-xyz">bar <span class="svelte-xyz">baz</span></p>
+<span class="svelte-xyz">buzz</span>

packages/svelte/tests/css/samples/not-selector-global/input.svelte

@@ -1,5 +1,9 @@
 <p class="foo">foo</p>
-<p class="bar">bar</p>
+<p class="bar">
+	bar
+	<span>baz</span>
+</p>
+<span>buzz</span>
 
 <style>
 	:not(:global(.foo)) {
@@ -12,12 +16,22 @@
 		color: green;
 	}
 	:global(.x) :not(p) {
-		color: red; /* TODO would be nice to prune this one day */
+		color: green;
 	}
 	:global(.x):not(p) {
-		color: red; /* TODO would be nice to prune this one day */
+		color: green;
 	}
 	:global(.x) :not(.unused) {
 		color: green;
 	}
+
+	:global(span):not(p span) {
+		color: green;
+	}
+	span:not(:global(p span)) {
+		color: green;
+	}
+	:global(span:not(p span)) {
+		color: green;
+	}
 </style>

packages/svelte/tests/css/samples/not-selector/_config.js

@@ -4,30 +4,30 @@ export default test({
 	warnings: [
 		{
 			code: 'css_unused_selector',
-			message: 'Unused CSS selector "p :not(.foo)"',
+			message: 'Unused CSS selector "span :not(.foo)"',
 			start: {
-				line: 22,
+				line: 26,
 				column: 1,
-				character: 291
+				character: 276
 			},
 			end: {
-				line: 22,
+				line: 26,
-				column: 13,
+				column: 16,
-				character: 303
+				character: 291
 			}
 		},
 		{
 			code: 'css_unused_selector',
-			message: 'Unused CSS selector "p :not(.unused)"',
+			message: 'Unused CSS selector "span :not(.unused)"',
 			start: {
-				line: 25,
+				line: 29,
 				column: 1,
-				character: 324
+				character: 312
 			},
 			end: {
-				line: 25,
+				line: 29,
-				column: 16,
+				column: 19,
-				character: 339
+				character: 330
 			}
 		}
 	]

packages/svelte/tests/css/samples/not-selector/expected.css

@@ -6,7 +6,7 @@
 		color: green;
 	}
 	.svelte-xyz:not(p) {
-		color: red; /* TODO would be nice to mark this as unused someday */
+		color: green;
 	}
 
 	.svelte-xyz:not(.foo):not(.unused) {
@@ -16,9 +16,13 @@
 	p.svelte-xyz:not(.foo) {
 		color: green;
 	}
-	/* (unused) p :not(.foo) {
+	/* (unused) span :not(.foo) {
 		color: red;
 	}*/
-	/* (unused) p :not(.unused) {
+	/* (unused) span :not(.unused) {
 		color: red;
 	}*/
+
+	span.svelte-xyz:not(p:where(.svelte-xyz) span:where(.svelte-xyz)) {
+		color: green;
+	}

packages/svelte/tests/css/samples/not-selector/expected.html

@@ -1 +1,3 @@
-<p class="foo svelte-xyz">foo</p> <p class="bar svelte-xyz">bar</p>
+<p class="foo svelte-xyz">foo</p>
+<p class="bar svelte-xyz">bar <span class="svelte-xyz">baz</span></p>
+<span class="svelte-xyz">buzz</span>

packages/svelte/tests/css/samples/not-selector/input.svelte

@@ -1,5 +1,9 @@
 <p class="foo">foo</p>
-<p class="bar">bar</p>
+<p class="bar">
+	bar
+	<span>baz</span>
+</p>
+<span>buzz</span>
 
 <style>
 	:not(.foo) {
@@ -9,7 +13,7 @@
 		color: green;
 	}
 	:not(p) {
-		color: red; /* TODO would be nice to mark this as unused someday */
+		color: green;
 	}
 
 	:not(.foo):not(.unused) {
@@ -19,10 +23,14 @@
 	p:not(.foo) {
 		color: green;
 	}
-	p :not(.foo) {
+	span :not(.foo) {
 		color: red;
 	}
-	p :not(.unused) {
+	span :not(.unused) {
 		color: red;
 	}
+
+	span:not(p span) {
+		color: green;
+	}
 </style>

packages/svelte/tests/css/test.ts

@@ -32,30 +32,17 @@ const { test, run } = suite<CssTest>(async (config, cwd) => {
 	await compile_directory(cwd, 'client', { cssHash: () => 'svelte-xyz', ...config.compileOptions });
 	await compile_directory(cwd, 'server', { cssHash: () => 'svelte-xyz', ...config.compileOptions });
 
-	const dom_css = fs.readFileSync(`${cwd}/_output/client/input.svelte.css`, 'utf-8').trim();
-	const ssr_css = fs.readFileSync(`${cwd}/_output/server/input.svelte.css`, 'utf-8').trim();
-
-	assert.equal(dom_css, ssr_css);
-
-	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(`${cwd}/expected.html`),
 		css: try_read_file(`${cwd}/expected.css`)
 	};
 
-	assert.equal(dom_css.trim().replace(/\r\n/g, '\n'), (expected.css ?? '').trim());
-
 	// we do this here, rather than in the expected.html !== null
 	// block, to verify that valid code was generated
 	const ClientComponent = (await import(`${cwd}/_output/client/input.svelte.js`)).default;
 	const ServerComponent = (await import(`${cwd}/_output/server/input.svelte.js`)).default;
 
-	// verify that the right elements have scoping selectors
+	// verify that the right elements have scoping selectors (do this first to ensure all actual files are written to disk)
 	if (expected.html !== null) {
 		const target = window.document.createElement('main');
 
@@ -75,6 +62,19 @@ const { test, run } = suite<CssTest>(async (config, cwd) => {
 		// const actual_ssr = ServerComponent.render(config.props).html;
 		// assert_html_equal(actual_ssr, expected.html);
 	}
+
+	const dom_css = fs.readFileSync(`${cwd}/_output/client/input.svelte.css`, 'utf-8').trim();
+	const ssr_css = fs.readFileSync(`${cwd}/_output/server/input.svelte.css`, 'utf-8').trim();
+
+	assert.equal(dom_css, ssr_css);
+
+	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);
+
+	assert.equal(dom_css.trim().replace(/\r\n/g, '\n'), (expected.css ?? '').trim());
 });
 
 export { test };