Right now createDocument returns an XMLDocument. It has this note:

This method creates an XMLDocument rather than a normal document. They are identical except for the addition of the load() method deployed content relies upon.

Since load() is gone as of whatwg/html@523f7a8, this note needs to be revised at the very least.

Edit: it appears @annevk already removed the note in 1595762 :)

I see two options:

• Make XMLDocument another alias of Document, just like HTMLDocument is currently specced to be. Remove all references to XMLDocument from DOM, and have HTML add a sentence about the alias.
• Keep the current empty-derived-interface setup in DOM. Maybe add some disparaging comments about this being a legacy feature.

Personally, I lean toward making it an alias, unless any implementers object. I think that the Document = HTMLDocument = XMLDocument story is a pretty good one, and in Blink at least, we're pretty close to implementing it---there are only a very small number of things left on HTMLDocument, none of which look very problematic. So I'd prefer to continue on that course.

There may be more history here (e.g. implementer objections) of which I am not aware.