LibWeb: Implement the :where() selector

This is identical to :is() except for specificity, so we can use the
same code paths. :^)

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

@@ -584,13 +584,17 @@ Result<Selector::SimpleSelector, Parser::ParsingResult> Parser::parse_simple_sel
             };
 
             auto& pseudo_function = pseudo_class_token.function();
-            if (pseudo_function.name().equals_ignoring_case("is"sv)) {
+            if (pseudo_function.name().equals_ignoring_case("is"sv)
-                simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Is;
+                || pseudo_function.name().equals_ignoring_case("where"sv)) {
+
+                simple_selector.pseudo_class.type = pseudo_function.name().equals_ignoring_case("is"sv)
+                    ? Selector::SimpleSelector::PseudoClass::Type::Is
+                    : Selector::SimpleSelector::PseudoClass::Type::Where;
                 auto function_token_stream = TokenStream(pseudo_function.values());
-                auto is_selector = parse_a_selector_list(function_token_stream, SelectorParsingMode::Forgiving);
+                auto argument_selector_list = parse_a_selector_list(function_token_stream, SelectorParsingMode::Forgiving);
                 // NOTE: Because it's forgiving, even complete garbage will parse OK as an empty selector-list.
-                VERIFY(!is_selector.is_error());
+                VERIFY(!argument_selector_list.is_error());
-                simple_selector.pseudo_class.argument_selector_list = is_selector.release_value();
+                simple_selector.pseudo_class.argument_selector_list = argument_selector_list.release_value();
             } else if (pseudo_function.name().equals_ignoring_case("not"sv)) {
                 simple_selector.pseudo_class.type = Selector::SimpleSelector::PseudoClass::Type::Not;
                 auto function_token_stream = TokenStream(pseudo_function.values());

Userland/Libraries/LibWeb/CSS/Selector.cpp

@@ -159,6 +159,7 @@ String Selector::SimpleSelector::serialize() const
         case Selector::SimpleSelector::PseudoClass::Type::NthLastChild:
         case Selector::SimpleSelector::PseudoClass::Type::Not:
         case Selector::SimpleSelector::PseudoClass::Type::Is:
+        case Selector::SimpleSelector::PseudoClass::Type::Where:
             // Otherwise, append ":" (U+003A), followed by the name of the pseudo-class, followed by "(" (U+0028),
             // followed by the value of the pseudo-class argument(s) determined as per below, followed by ")" (U+0029), to s.
             s.append(':');
@@ -169,9 +170,10 @@ String Selector::SimpleSelector::serialize() const
                 // The result of serializing the value using the rules to serialize an <an+b> value.
                 s.append(pseudo_class.nth_child_pattern.serialize());
             } else if (pseudo_class.type == Selector::SimpleSelector::PseudoClass::Type::Not
-                || pseudo_class.type == Selector::SimpleSelector::PseudoClass::Type::Is) {
+                || pseudo_class.type == Selector::SimpleSelector::PseudoClass::Type::Is
+                || pseudo_class.type == Selector::SimpleSelector::PseudoClass::Type::Where) {
                 // The result of serializing the value using the rules for serializing a group of selectors.
-                // NOTE: `:is()` isn't in the spec for this yet, but it should be!
+                // NOTE: `:is()` and `:where()` aren't in the spec for this yet, but it should be!
                 s.append(serialize_a_group_of_selectors(pseudo_class.argument_selector_list));
             }
             s.append(')');

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -77,6 +77,7 @@ public:
                 Checked,
                 Is,
                 Not,
+                Where,
                 Active,
             };
             Type type { Type::None };
@@ -216,6 +217,8 @@ constexpr StringView pseudo_class_name(Selector::SimpleSelector::PseudoClass::Ty
         return "is"sv;
     case Selector::SimpleSelector::PseudoClass::Type::Not:
         return "not"sv;
+    case Selector::SimpleSelector::PseudoClass::Type::Where:
+        return "where"sv;
     case Selector::SimpleSelector::PseudoClass::Type::None:
         break;
     }

Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -155,6 +155,7 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Checked:
         return matches_checked_pseudo_class(element);
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Is:
+    case CSS::Selector::SimpleSelector::PseudoClass::Type::Where:
         for (auto& selector : pseudo_class.argument_selector_list) {
             if (matches(selector, element))
                 return true;

Userland/Libraries/LibWeb/Dump.cpp

@@ -433,11 +433,15 @@ void dump_selector(StringBuilder& builder, CSS::Selector const& selector)
                 case CSS::Selector::SimpleSelector::PseudoClass::Type::Is:
                     pseudo_class_description = "Is";
                     break;
+                case CSS::Selector::SimpleSelector::PseudoClass::Type::Where:
+                    pseudo_class_description = "Where";
+                    break;
                 }
 
                 builder.appendff(" pseudo_class={}", pseudo_class_description);
                 if (pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::Not
-                    || pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::Is) {
+                    || pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::Is
+                    || pseudo_class.type == CSS::Selector::SimpleSelector::PseudoClass::Type::Where) {
                     builder.append("([");
                     for (auto& selector : pseudo_class.argument_selector_list)
                         dump_selector(builder, selector);