LibWeb: Add HTMLDocument class

This class is currently not in the spec, but it *is* still in all the
major browser engines. For compatibility reasons, let's do what other
engines do.

There is discussion about bringing HTMLDocument back into specs:
- https://github.com/whatwg/html/issues/4792
- https://github.com/whatwg/dom/issues/221

Userland/Libraries/LibWeb/HTML/HTMLDocument.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/Document.h>
+
+namespace Web::HTML {
+
+// NOTE: This class is not currently in the specifications but it *is* implemented by all major browsers.
+//       There is discussion about bringing it back:
+//       https://github.com/whatwg/html/issues/4792
+//       https://github.com/whatwg/dom/issues/221
+class HTMLDocument final : public DOM::Document {
+    JS_CELL(HTMLDocument, DOM::Document);
+
+public:
+    virtual ~HTMLDocument() override;
+
+    static WebIDL::ExceptionOr<JS::NonnullGCPtr<HTMLDocument>> create(JS::Realm&, AK::URL const& url = "about:blank"sv);
+    WebIDL::ExceptionOr<JS::NonnullGCPtr<HTMLDocument>> construct_impl(JS::Realm&);
+
+private:
+    HTMLDocument(JS::Realm&, AK::URL const&);
+};
+
+}