LibWeb/CSS: Correct parsing of @supports selector()

A couple of fixes here:
- Parse a `<complex-selector>` instead of a `<selector-list>`
- Don't match if any unknown `::-webkit-*` pseudo-elements are found

Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -304,9 +304,13 @@ OwnPtr<BooleanExpression> Parser::parse_supports_feature(TokenStream<ComponentVa
             builder.append(item.to_string());
         transaction.commit();
         TokenStream selector_tokens { first_token.function().value };
-        return Supports::Selector::create(
-            builder.to_string_without_validation(),
-            !parse_a_selector_list(selector_tokens, SelectorType::Standalone).is_error());
+        auto maybe_selector = parse_complex_selector(selector_tokens, SelectorType::Standalone);
+        // A CSS processor is considered to support a CSS selector if it accepts that all aspects of that selector,
+        // recursively, (rather than considering any of its syntax to be unknown or invalid) and that selector doesn’t
+        // contain unknown -webkit- pseudo-elements.
+        // https://drafts.csswg.org/css-conditional-4/#dfn-support-selector
+        bool matches = !maybe_selector.is_error() && !maybe_selector.value()->contains_unknown_webkit_pseudo_element();
+        return Supports::Selector::create(builder.to_string_without_validation(), matches);
     }
 
     // `<supports-font-tech-fn> = font-tech( <font-tech> )`

Libraries/LibWeb/CSS/Selector.cpp

@@ -646,6 +646,24 @@ NonnullRefPtr<Selector> Selector::relative_to(SimpleSelector const& parent) cons
     return Selector::create(move(copied_compound_selectors));
 }
 
+bool Selector::contains_unknown_webkit_pseudo_element() const
+{
+    for (auto const& compound_selector : m_compound_selectors) {
+        for (auto const& simple_selector : compound_selector.simple_selectors) {
+            if (simple_selector.type == SimpleSelector::Type::PseudoClass) {
+                for (auto const& child_selector : simple_selector.pseudo_class().argument_selector_list) {
+                    if (child_selector->contains_unknown_webkit_pseudo_element()) {
+                        return true;
+                    }
+                }
+            }
+            if (simple_selector.type == SimpleSelector::Type::PseudoElement && simple_selector.pseudo_element().type() == PseudoElement::Type::UnknownWebKit)
+                return true;
+        }
+    }
+    return false;
+}
+
 RefPtr<Selector> Selector::absolutized(Selector::SimpleSelector const& selector_for_nesting) const
 {
     if (!contains_the_nesting_selector())

Libraries/LibWeb/CSS/Selector.h

@@ -262,6 +262,7 @@ public:
     NonnullRefPtr<Selector> relative_to(SimpleSelector const&) const;
     bool contains_the_nesting_selector() const { return m_contains_the_nesting_selector; }
     bool contains_hover_pseudo_class() const { return m_contains_hover_pseudo_class; }
+    bool contains_unknown_webkit_pseudo_element() const;
     RefPtr<Selector> absolutized(SimpleSelector const& selector_for_nesting) const;
     u32 specificity() const;
     String serialize() const;