LibWeb: Make sure that head and body always get the HTML element

Now that document element returns a generic DOM element, we need to
make sure head and body get a html element. 

The spec just says to check if the document element is a html element,
so let's do that.

Libraries/LibWeb/DOM/Document.h

@@ -86,6 +86,7 @@ public:
     const Node* inspected_node() const { return m_inspected_node; }
 
     const Element* document_element() const;
+    const HTML::HTMLHtmlElement* html_element() const;
     const HTML::HTMLHeadElement* head() const;
     const HTML::HTMLElement* body() const;