LibWeb: Add the Document.documentElement API

Also change DOM::Document::document_element() to return an Element*
and not an HTML::HTMLHtmlElement since that's not the only kind of
documentElement we might encounter.

Libraries/LibWeb/DOM/Document.cpp

@@ -127,9 +127,9 @@ void Document::fixup()
     this->append_child(html);
 }
 
-const HTML::HTMLHtmlElement* Document::document_element() const
+const Element* Document::document_element() const
 {
-    return first_child_of_type<HTML::HTMLHtmlElement>();
+    return first_child_of_type<Element>();
 }
 
 const HTML::HTMLHeadElement* Document::head() const

Libraries/LibWeb/DOM/Document.h

@@ -85,7 +85,7 @@ public:
     Node* inspected_node() { return m_inspected_node; }
     const Node* inspected_node() const { return m_inspected_node; }
 
-    const HTML::HTMLHtmlElement* document_element() const;
+    const Element* document_element() const;
     const HTML::HTMLHeadElement* head() const;
     const HTML::HTMLElement* body() const;
 

Libraries/LibWeb/DOM/Document.idl

@@ -9,6 +9,7 @@ interface Document : Node {
     readonly attribute DOMString compatMode;
     readonly attribute DocumentType? doctype;
 
+    readonly attribute Element? documentElement;
     readonly attribute HTMLElement? body;
 
 }

Libraries/LibWeb/Tests/Document/documentElement.js

@@ -0,0 +1,8 @@
+loadPage("file:///res/html/misc/blank.html");
+
+afterInitialPageLoad(() => {
+    test("Basic functionality", () => {
+        expect(document.documentElement).not.toBe(null);
+        expect(document.documentElement.nodeName).toBe("html");
+    });
+});