LibHTML: Add a StyledNode class.

I'd like to try doing DOM -> style tree -> layout tree. I'm not exactly sure
how it's gonna work, but we'll figure it out as we go.

LibHTML/CSS/StyledNode.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <AK/HashMap.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/AKString.h>
+#include <LibHTML/TreeNode.h>
+#include <LibHTML/CSS/StyleValue.h>
+
+class Node;
+
+class StyledNode : public TreeNode<StyledNode> {
+public:
+    ~StyledNode();
+
+    const Node* node() const { return m_node; }
+
+    template<typename Callback>
+    inline void for_each_child(Callback callback) const
+    {
+        for (auto* node = first_child(); node; node = node->next_sibling())
+            callback(*node);
+    }
+
+    template<typename Callback>
+    inline void for_each_child(Callback callback)
+    {
+        for (auto* node = first_child(); node; node = node->next_sibling())
+            callback(*node);
+    }
+
+protected:
+    explicit StyledNode(const Node*);
+
+private:
+    const Node* m_node { nullptr };
+    HashMap<String, NonnullRefPtr<StyleValue>> m_property_values;
+};