LibWeb: Make CSS :empty selector match empty text nodes as well

Previously we were only matching elements with *no* text children.
With this patch, we now also allow any number of empty text children to
be present as well.

1% progression on ACID3. :^)

Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -93,8 +93,23 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         return !element.next_element_sibling();
     case CSS::Selector::SimpleSelector::PseudoClass::Type::OnlyChild:
         return !(element.previous_element_sibling() || element.next_element_sibling());
-    case CSS::Selector::SimpleSelector::PseudoClass::Type::Empty:
-        return !(element.first_child_of_type<DOM::Element>() || element.first_child_of_type<DOM::Text>());
+    case CSS::Selector::SimpleSelector::PseudoClass::Type::Empty: {
+        if (!element.has_children())
+            return true;
+        if (element.first_child_of_type<DOM::Element>())
+            return false;
+        // NOTE: CSS Selectors level 4 changed ":empty" to also match whitespace-only text nodes.
+        //       However, none of the major browser supports this yet, so let's just hang back until they do.
+        bool has_nonempty_text_child = false;
+        element.for_each_child_of_type<DOM::Text>([&](auto const& text_child) {
+            if (!text_child.data().is_empty()) {
+                has_nonempty_text_child = true;
+                return IterationDecision::Break;
+            }
+            return IterationDecision::Continue;
+        });
+        return !has_nonempty_text_child;
+    }
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Root:
         return is<HTML::HTMLHtmlElement>(element);
     case CSS::Selector::SimpleSelector::PseudoClass::Type::FirstOfType: