LibWeb/CSS: Parse the ::slotted pseudo-element

Author: https://github.com/shannonbooth
Commit: 9054ff29f0
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5414
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/Selector.cpp

@@ -294,6 +294,11 @@ String Selector::PseudoElementSelector::serialize() const
     }
 
     m_value.visit(
+        [&builder](NonnullRefPtr<Selector> const& compund_selector) {
+            builder.append('(');
+            builder.append(compund_selector->serialize());
+            builder.append(')');
+        },
         [&builder](PTNameSelector const& pt_name_selector) {
             builder.append('(');
             if (pt_name_selector.is_universal)