[dom] Clarify what Document constructor does? (#137)

Hi everyone and merry xmas!
What is supposed to do the Document constructor? (I'm likely failing at understanding)

> The Document() constructor must return a new document whose origin is an alias to the origin of the global object’s associated document, and effective script origin is an alias to the effective script origin of the global object’s associated document [HTML]
> Unlike createDocument() this constructor does not return an XMLDocument object, but a document (Document object)

If it's not a XMLDocument, is it an HTML Document, then? Or is it an xml document which was just constructed differently? (XMLDocument doesn't add members to the interface)
basically, what are its "type" and "mode" properties set to?

The HTML spec adds functionality to Document but it contains members that are only suited to HTML Documents
Also, in the HTML spec `windowHTMLDocument` is just an alias for `windowDocument`, but all UAs have an actual `HTMLDocument` class (as in DOM2, with the exception of MSIE9, which only has `Document` Newer MS browsers all have `HTMLDocument : Document`)

Thank you in advance

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/137