LibHTML: Start fleshing out a StyleResolver class.

This will be responsible for matching selectors and creating LayoutStyle
objects for the document and its elements.

LibHTML/CSS/StyleResolver.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <AK/OwnPtr.h>
+#include <AK/NonnullRefPtrVector.h>
+#include <LibHTML/Layout/LayoutStyle.h>
+
+class Document;
+class Element;
+class StyleSheet;
+
+class StyleResolver {
+public:
+    explicit StyleResolver(Document&);
+    ~StyleResolver();
+
+    Document& document() { return m_document; }
+    const Document& document() const { return m_document; }
+
+    void add_sheet(const StyleSheet& sheet) { m_sheets.append(sheet); }
+
+    OwnPtr<LayoutStyle> resolve_element_style(const Element&);
+    OwnPtr<LayoutStyle> resolve_document_style(const Document&);
+
+private:
+    Document& m_document;
+
+    NonnullRefPtrVector<StyleSheet> m_sheets;
+};