LibWeb/CSS: Implement import at-rule supports conditions

This indicates the features the browser must support for the given
stylesheet to be fetched.

Libraries/LibWeb/CSS/CSSImportRule.cpp

@@ -23,16 +23,17 @@ namespace Web::CSS {
 
 GC_DEFINE_ALLOCATOR(CSSImportRule);
 
-GC::Ref<CSSImportRule> CSSImportRule::create(URL::URL url, DOM::Document& document)
+GC::Ref<CSSImportRule> CSSImportRule::create(URL::URL url, DOM::Document& document, RefPtr<Supports> supports)
 {
     auto& realm = document.realm();
-    return realm.create<CSSImportRule>(move(url), document);
+    return realm.create<CSSImportRule>(move(url), document, supports);
 }
 
-CSSImportRule::CSSImportRule(URL::URL url, DOM::Document& document)
+CSSImportRule::CSSImportRule(URL::URL url, DOM::Document& document, RefPtr<Supports> supports)
     : CSSRule(document.realm(), Type::Import)
     , m_url(move(url))
     , m_document(document)
+    , m_supports(supports)
 {
 }
 
@@ -70,6 +71,11 @@ String CSSImportRule::serialized() const
     // 2. The result of performing serialize a URL on the rule’s location.
     serialize_a_url(builder, m_url.to_string());
 
+    // AD-HOC: Serialize the rule's supports condition if it exists.
+    //         This isn't currently specified, but major browsers include this in their serialization of import rules
+    if (m_supports)
+        builder.appendff(" supports({})", m_supports->to_string());
+
     // FIXME: 3. If the rule’s associated media list is not empty, a single SPACE (U+0020) followed by the result of performing serialize a media query list on the media list.
 
     // 4. The string ";", i.e., SEMICOLON (U+003B).
@@ -88,7 +94,10 @@ void CSSImportRule::fetch()
     VERIFY(parent_style_sheet());
     auto& parent_style_sheet = *this->parent_style_sheet();
 
-    // FIXME: 2. If rule has a <supports-condition>, and that condition is not true, return.
+    // 2. If rule has a <supports-condition>, and that condition is not true, return.
+    if (m_supports && !m_supports->matches()) {
+        return;
+    }
 
     // 3. Let parsedUrl be the result of the URL parser steps with rule’s URL and parentStylesheet’s location.
     //    If the algorithm returns an error, return. [CSSOM]

Libraries/LibWeb/CSS/CSSImportRule.h

@@ -21,7 +21,7 @@ class CSSImportRule final
     GC_DECLARE_ALLOCATOR(CSSImportRule);
 
 public:
-    [[nodiscard]] static GC::Ref<CSSImportRule> create(URL::URL, DOM::Document&);
+    [[nodiscard]] static GC::Ref<CSSImportRule> create(URL::URL, DOM::Document&, RefPtr<Supports>);
 
     virtual ~CSSImportRule() = default;
 
@@ -34,7 +34,7 @@ public:
     CSSStyleSheet* style_sheet_for_bindings() { return m_style_sheet; }
 
 private:
-    CSSImportRule(URL::URL, DOM::Document&);
+    CSSImportRule(URL::URL, DOM::Document&, RefPtr<Supports>);
 
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
@@ -48,6 +48,7 @@ private:
 
     URL::URL m_url;
     GC::Ptr<DOM::Document> m_document;
+    RefPtr<Supports> m_supports;
     GC::Ptr<CSSStyleSheet> m_style_sheet;
     Optional<DOM::DocumentLoadEventDelayer> m_document_load_event_delayer;
 };

Libraries/LibWeb/CSS/Parser/RuleParsing.cpp

@@ -161,16 +161,35 @@ GC::Ptr<CSSImportRule> Parser::convert_to_import_rule(AtRule const& rule)
     }
 
     tokens.discard_whitespace();
-    // TODO: Support layers and import-conditions
+    // FIXME: Implement layer support.
+    RefPtr<Supports> supports {};
+    if (tokens.next_token().is_function("supports"sv)) {
+        auto component_value = tokens.consume_a_token();
+        TokenStream supports_tokens { component_value.function().value };
+        if (supports_tokens.next_token().is_block()) {
+            supports = parse_a_supports(supports_tokens);
+        } else {
+            m_rule_context.append(ContextType::SupportsCondition);
+            auto declaration = consume_a_declaration(supports_tokens);
+            m_rule_context.take_last();
+            if (declaration.has_value()) {
+                auto supports_declaration = Supports::Declaration::create(declaration->to_string(), convert_to_style_property(*declaration).has_value());
+                supports = Supports::create(supports_declaration.release_nonnull<BooleanExpression>());
+            }
+        }
+    }
+
+    // FIXME: Implement media query support.
+
     if (tokens.has_next_token()) {
         if constexpr (CSS_PARSER_DEBUG) {
-            dbgln("Failed to parse @import rule: Trailing tokens after URL are not yet supported.");
+            dbgln("Failed to parse @import rule:");
             tokens.dump_all_tokens();
         }
         return {};
     }
 
-    return CSSImportRule::create(url.value(), const_cast<DOM::Document&>(*document()));
+    return CSSImportRule::create(url.value(), const_cast<DOM::Document&>(*document()), supports);
 }
 
 Optional<FlyString> Parser::parse_layer_name(TokenStream<ComponentValue>& tokens, AllowBlankLayerName allow_blank_layer_name)