some progress

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

@@ -167,7 +167,17 @@ const visitors = {
 			state.code.appendLeft(last, '*/');
 		}
 
-		const specificity = path.at(-1)?.type === 'Rule' ? { bumped: false } : state.specificity;
+		// if we're in a `:is(...)` or whatever, keep existing specificity bump state
+		let specificity = state.specificity;
+
+		// if this selector list belongs to a rule, require a specificity bump for the
+		// first scoped selector but only if we're at the top level
+		// TODO this isn't quite right, it would break with `:global(x) {...}`
+		const parent = path.at(-1);
+		if (parent?.type === 'Rule') {
+			specificity = { bumped: !!parent.metadata.parent_rule };
+		}
+
 		next({ ...state, specificity });
 	},
 	ComplexSelector(node, context) {

packages/svelte/tests/css/samples/nested-css/expected.css

@@ -13,13 +13,13 @@
 			/* (unused) .unused {
 				color: red;
 
-				.c:where(.svelte-xyz) {
+				.c {
 					color: red;
 				}
 			}*/
 		}
 
-		/* (empty) .d:where(.svelte-xyz) {
+		/* (empty) .d {
 			.unused {
 				color: red;
 			}
@@ -29,7 +29,7 @@
 		& .b:where(.svelte-xyz) {
 			color: green;
 
-			/* (empty) .c:where(.svelte-xyz) {
+			/* (empty) .c {
 				& & {
 					color: red;
 				}
@@ -42,7 +42,7 @@
 
 		/* silly but valid */
 		&& {
-			color: rebeccapurple;
+			color: green;
 		}
 
 		.container:where(.svelte-xyz) & {

packages/svelte/tests/css/samples/nested-css/input.svelte

@@ -56,7 +56,7 @@
 
 		/* silly but valid */
 		&& {
-			color: rebeccapurple;
+			color: green;
 		}
 
 		.container & {