LibWeb: Make event dispatching spec-compliant

Specification: https://dom.spec.whatwg.org/#concept-event-dispatch

This also introduces shadow roots due to it being a requirement of
the event dispatcher.

However, it does not introduce the full shadow DOM, that can be
left for future work.

This changes some event dispatches which require certain attributes
to be initialised to a value.

Libraries/LibWeb/DOM/DOMImplementation.cpp

@@ -44,6 +44,7 @@ const NonnullRefPtr<Document> DOMImplementation::create_htmldocument(const Strin
     auto html_document = Document::create();
 
     html_document->set_content_type("text/html");
+    html_document->set_ready_for_post_load_tasks(true);
 
     auto doctype = adopt(*new DocumentType(html_document));
     doctype->set_name("html");