LibWeb: Implement the :defined pseudo class

This selects an element if it is either a built-in element, or an
upgraded custom element.

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

@@ -494,6 +494,8 @@ Parser::ParseErrorOr<Selector::SimpleSelector> Parser::parse_pseudo_simple_selec
             return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Checked);
         if (pseudo_name.equals_ignoring_ascii_case("indeterminate"sv))
             return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Indeterminate);
+        if (pseudo_name.equals_ignoring_ascii_case("defined"sv))
+            return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Defined);
         if (pseudo_name.equals_ignoring_ascii_case("disabled"sv))
             return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Disabled);
         if (pseudo_name.equals_ignoring_ascii_case("empty"sv))

Userland/Libraries/LibWeb/CSS/Selector.cpp

@@ -228,6 +228,7 @@ ErrorOr<String> Selector::SimpleSelector::serialize() const
         case Selector::SimpleSelector::PseudoClass::Type::Checked:
         case Selector::SimpleSelector::PseudoClass::Type::Active:
         case Selector::SimpleSelector::PseudoClass::Type::Scope:
+        case Selector::SimpleSelector::PseudoClass::Type::Defined:
             // If the pseudo-class does not accept arguments append ":" (U+003A), followed by the name of the pseudo-class, to s.
             TRY(s.try_append(':'));
             TRY(s.try_append(pseudo_class_name(pseudo_class.type)));

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -112,6 +112,7 @@ public:
                 Active,
                 Lang,
                 Scope,
+                Defined,
             };
             Type type;
 
@@ -295,6 +296,8 @@ constexpr StringView pseudo_class_name(Selector::SimpleSelector::PseudoClass::Ty
         return "lang"sv;
     case Selector::SimpleSelector::PseudoClass::Type::Scope:
         return "scope"sv;
+    case Selector::SimpleSelector::PseudoClass::Type::Defined:
+        return "defined"sv;
     }
     VERIFY_NOT_REACHED();
 }

Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -268,6 +268,8 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         return matches_checked_pseudo_class(element);
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Indeterminate:
         return matches_indeterminate_pseudo_class(element);
+    case CSS::Selector::SimpleSelector::PseudoClass::Type::Defined:
+        return element.is_defined();
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Is:
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Where:
         for (auto& selector : pseudo_class.argument_selector_list) {

Userland/Libraries/LibWeb/Dump.cpp

@@ -459,6 +459,9 @@ void dump_selector(StringBuilder& builder, CSS::Selector const& selector)
                 case CSS::Selector::SimpleSelector::PseudoClass::Type::Scope:
                     pseudo_class_description = "Scope";
                     break;
+                case CSS::Selector::SimpleSelector::PseudoClass::Type::Defined:
+                    pseudo_class_description = "Defined";
+                    break;
                 }
 
                 builder.appendff(" pseudo_class={}", pseudo_class_description);