LibWeb: Parse the ::backdrop pseudo element

We don't do anything with this yet, but let's at least not reject
selectors with ::backdrop in the parser.

Userland/Libraries/LibWeb/CSS/Selector.cpp

@@ -394,6 +394,8 @@ StringView Selector::PseudoElement::name(Selector::PseudoElement::Type pseudo_el
         return "-webkit-slider-runnable-track"sv;
     case Selector::PseudoElement::Type::SliderThumb:
         return "-webkit-slider-thumb"sv;
+    case Selector::PseudoElement::Type::Backdrop:
+        return "backdrop"sv;
     case Selector::PseudoElement::Type::KnownPseudoElementCount:
         break;
     case Selector::PseudoElement::Type::UnknownWebKit:
@@ -430,6 +432,8 @@ Optional<Selector::PseudoElement> Selector::PseudoElement::from_string(FlyString
         return Selector::PseudoElement { Selector::PseudoElement::Type::Placeholder };
     } else if (name.equals_ignoring_ascii_case("selection"sv)) {
         return Selector::PseudoElement { Selector::PseudoElement::Type::Selection };
+    } else if (name.equals_ignoring_ascii_case("backdrop"sv)) {
+        return Selector::PseudoElement { Selector::PseudoElement::Type::Backdrop };
     } else if (name.equals_ignoring_ascii_case("-webkit-slider-runnable-track"sv)) {
         return Selector::PseudoElement { Selector::PseudoElement::Type::SliderRunnableTrack };
     } else if (name.equals_ignoring_ascii_case("-webkit-slider-thumb"sv)) {

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -39,6 +39,7 @@ public:
             Selection,
             SliderRunnableTrack,
             SliderThumb,
+            Backdrop,
 
             // Keep this last.
             KnownPseudoElementCount,