LibWeb: Make :host() take a <compound-selector>

This matches the spec:
https://drafts.csswg.org/css-scoping-1/#selectordef-host

Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateCSSPseudoClass.cpp

@@ -71,6 +71,7 @@ struct PseudoClassMetadata {
         None,
         ANPlusB,
         ANPlusBOf,
+        CompoundSelector,
         ForgivingSelectorList,
         LanguageRanges,
         SelectorList,
@@ -164,6 +165,8 @@ PseudoClassMetadata pseudo_class_metadata(PseudoClass pseudo_class)
                 parameter_type = "ANPlusB"_string;
             } else if (argument_string == "<an+b-of>"sv) {
                 parameter_type = "ANPlusBOf"_string;
+            } else if (argument_string == "<compound-selector>"sv) {
+                parameter_type = "CompoundSelector"_string;
             } else if (argument_string == "<forgiving-selector-list>"sv) {
                 parameter_type = "ForgivingSelectorList"_string;
             } else if (argument_string == "<language-ranges>"sv) {

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -633,6 +633,24 @@ Parser::ParseErrorOr<Selector::SimpleSelector> Parser::parse_pseudo_simple_selec
             return parse_nth_child_selector(pseudo_class, pseudo_function.values(), false);
         case PseudoClassMetadata::ParameterType::ANPlusBOf:
             return parse_nth_child_selector(pseudo_class, pseudo_function.values(), true);
+        case PseudoClassMetadata::ParameterType::CompoundSelector: {
+            auto function_token_stream = TokenStream(pseudo_function.values());
+            auto compound_selector = MUST(parse_compound_selector(function_token_stream));
+            if (!compound_selector.has_value()) {
+                dbgln_if(CSS_PARSER_DEBUG, "Failed to parse :{}() parameter as a compound selector", pseudo_function.name());
+                return ParseError::SyntaxError;
+            }
+
+            Vector compound_selectors { compound_selector.release_value() };
+            auto selector = Selector::create(move(compound_selectors));
+
+            return Selector::SimpleSelector {
+                .type = Selector::SimpleSelector::Type::PseudoClass,
+                .value = Selector::SimpleSelector::PseudoClassSelector {
+                    .type = pseudo_class,
+                    .argument_selector_list = { move(selector) } }
+            };
+        }
         case PseudoClassMetadata::ParameterType::ForgivingSelectorList: {
             auto function_token_stream = TokenStream(pseudo_function.values());
             // NOTE: Because it's forgiving, even complete garbage will parse OK as an empty selector-list.

Userland/Libraries/LibWeb/CSS/PseudoClasses.json

@@ -36,7 +36,7 @@
     "argument": ""
   },
   "host": {
-    "argument": "<selector-list>?"
+    "argument": "<compound-selector>?"
   },
   "hover": {
     "argument": ""

Userland/Libraries/LibWeb/Dump.cpp

@@ -496,6 +496,7 @@ void dump_selector(StringBuilder& builder, CSS::Selector const& selector)
                     builder.append(")"sv);
                     break;
                 }
+                case CSS::PseudoClassMetadata::ParameterType::CompoundSelector:
                 case CSS::PseudoClassMetadata::ParameterType::ForgivingSelectorList:
                 case CSS::PseudoClassMetadata::ParameterType::SelectorList: {
                     builder.append("(["sv);