LibWeb/CSS: Implement the :state(foo) pseudo-class

This matches custom elements that have `foo` in their custom states set.

The 2 test failures here are because we don't support `::part()` yet.

Libraries/LibWeb/CSS/PseudoClasses.json

@@ -155,6 +155,9 @@
   "stalled": {
     "argument": ""
   },
+  "state": {
+    "argument": "<ident>"
+  },
   "suboptimal-value": {
     "argument": ""
   },

Libraries/LibWeb/CSS/Selector.cpp

@@ -61,6 +61,7 @@ static bool can_selector_use_fast_matches(Selector const& selector)
                         PseudoClass::LocalLink,
                         PseudoClass::OnlyChild,
                         PseudoClass::Root,
+                        PseudoClass::State,
                         PseudoClass::Visited))
                     return false;
             } else if (!first_is_one_of(simple_selector.type,

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -15,6 +15,7 @@
 #include <LibWeb/DOM/NamedNodeMap.h>
 #include <LibWeb/DOM/Text.h>
 #include <LibWeb/HTML/AttributeNames.h>
+#include <LibWeb/HTML/CustomElements/CustomStateSet.h>
 #include <LibWeb/HTML/HTMLAnchorElement.h>
 #include <LibWeb/HTML/HTMLDetailsElement.h>
 #include <LibWeb/HTML/HTMLDialogElement.h>
@@ -1059,6 +1060,15 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
 
         return false;
     }
+    case CSS::PseudoClass::State: {
+        // https://html.spec.whatwg.org/multipage/semantics-other.html#selector-custom
+        // The :state(identifier) pseudo-class must match all custom elements whose states set's set entries contains identifier.
+        if (!element.is_custom())
+            return false;
+        if (auto* custom_state_set = element.custom_state_set())
+            return custom_state_set->has_state(pseudo_class.ident->string_value);
+        return false;
+    }
     }
 
     return false;