LibHTML: Implement some very simple selector matching.

We walk the entire DOM and check all selectors against all elements. Only
id, class and tag name are checked right now. There's no ancestor stack
or compound selectors. All in good time :^)

LibHTML/test.cpp

@@ -3,6 +3,7 @@
 #include <LibHTML/Frame.h>
 #include <LibHTML/Parser/CSSParser.h>
 #include <LibHTML/CSS/StyleResolver.h>
+#include <LibHTML/DOM/Element.h>
 #include <LibHTML/Parser/HTMLParser.h>
 #include <stdio.h>
 
@@ -29,6 +30,17 @@ int main(int argc, char** argv)
 
     auto doc_style = resolver.resolve_document_style(*doc);
 
+    Function<void(const ParentNode&)> resolve_style = [&](const ParentNode& node) {
+        node.for_each_child([&](const Node& child) {
+            if (!child.is_element())
+                return;
+            auto style = resolver.resolve_element_style(static_cast<const Element&>(node));
+            printf("Resolved LayoutStyle{%p} for Element{%p}\n", style.ptr(), &node);
+            resolve_style(static_cast<const Element&>(child));
+        });
+    };
+    resolve_style(*doc);
+
     doc->build_layout_tree();
     ASSERT(doc->layout_node());