Issue #1592 - Part 2: Parse ::slotted() pseudo-element as if it were a pseudo-class

- Block slot elements from being matched by ::slotted
- Ensure ::slotted() is serialized as a pseudo-element
- Add pref to control whether the pseudo-class is enabled

layout/style/nsCSSRuleProcessor.cpp

@@ -1963,6 +1963,29 @@ static bool SelectorMatches(Element* aElement,
         }
         break;
 
+      case CSSPseudoClassType::slotted:
+        {
+          // Slot elements cannot be matched.
+          if (aElement->IsHTMLElement(nsGkAtoms::slot)) {
+            return false;
+          }
+
+          // The current element must have an assigned slot.
+          if (!aElement->GetAssignedSlot()) {
+            return false;
+          }
+
+          NodeMatchContext nodeContext(EventStates(),
+                                       aNodeMatchContext.mIsRelevantLink);
+          if (!SelectorListMatches(aElement,
+                                   pseudoClass,
+                                   nodeContext,
+                                   aTreeMatchContext)) {
+            return false;
+          }
+        }
+        break;
+
       case CSSPseudoClassType::host:
         {
           ShadowRoot* shadow = aElement->GetShadowRoot();