LibHTML: Add the outline of a CSS stylesheet object graph.

LibHTML/CSS/StyleValue.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <AK/Retainable.h>
+
+class StyleValue : public Retainable<StyleValue> {
+public:
+    virtual ~StyleValue();
+
+    enum Type {
+        Invalid,
+        Inherit,
+        Initial,
+        Primitive,
+    };
+
+    Type type() const { return m_type; }
+
+protected:
+    explicit StyleValue(Type);
+
+private:
+    Type m_type { Type::Invalid };
+};