fallback unknown rule to new unknown type

src/browser/tests/css/stylesheet.html

@@ -668,3 +668,13 @@
     testing.expectTrue(exists);
 }
 </script>
+
+<script id="CSSStyleSheet_insertRule_unknown_at_rule_typed_as_zero">
+{
+  const sheet = new CSSStyleSheet();
+  sheet.insertRule('@layer utilities { .x { color: red; } }');
+  testing.expectEqual(1, sheet.cssRules.length);
+  testing.expectEqual(0, sheet.cssRules[0].type);
+  testing.expectTrue(sheet.cssRules[0].cssText.includes('@layer'));
+}
+</script>

src/browser/webapi/css/CSSRule.zig

@@ -23,6 +23,7 @@ pub const Type = union(enum) {
     font_feature_values: void,
     viewport: void,
     region_style: void,
+    unknown: void,
 };
 
 _type: Type,
@@ -62,6 +63,9 @@ pub fn initAtRule(rule_type: Type, text: []const u8, frame: *Frame) !*CSSRule {
 }
 
 pub fn getType(self: *const CSSRule) u16 {
+    if (self._type == .unknown) {
+        return 0;
+    }
     return @as(u16, @intFromEnum(std.meta.activeTag(self._type))) + 1;
 }
 

src/browser/webapi/css/CSSStyleSheet.zig

@@ -124,12 +124,7 @@ pub fn insertRule(self: *CSSStyleSheet, rule: []const u8, maybe_index: ?u32, fra
 /// Map an at-rule keyword (without `@`) to the matching `CSSRule.Type`
 /// variant. Vendor prefixes are stripped before matching
 /// (`-webkit-keyframes` -> `keyframes`). Unrecognized keywords fall back
-/// to `.media`: this is a deliberate choice to avoid changing the
-/// `CSSRule.Type` enum's numeric layout (which is exposed as the spec
-/// `CSSRule.type` constant) just to add an `unknown` variant. The CSS
-/// engine doesn't use the type for anything; the value matters only for
-/// JS-side `rule.type` checks, and CSS-in-JS dedup paths key on `length`
-/// or `cssText` rather than `type`.
+/// to `.unknown`: which maps to 0 for rule.type.
 fn atRuleTypeFor(keyword_with_prefix: []const u8) CSSRule.Type {
     var keyword = keyword_with_prefix;
     inline for (.{ "-webkit-", "-moz-", "-ms-", "-o-" }) |prefix| {
@@ -152,7 +147,7 @@ fn atRuleTypeFor(keyword_with_prefix: []const u8) CSSRule.Type {
     if (eql(keyword, "font-feature-values")) return .font_feature_values;
     if (eql(keyword, "viewport")) return .viewport;
     if (eql(keyword, "document")) return .document;
-    return .media;
+    return .unknown;
 }
 
 pub fn deleteRule(self: *CSSStyleSheet, index: u32, frame: *Frame) !void {