To summarize options:
1) Require any non-presented DocumentWindow to have a null defaultView.
2) Allow a non-presented DocumentWindow to have any AbstractView or null
as the defaultView (essentially, we disclaim stating a requirement on
non-presented documents).
3) Forbid any non-presented Document to implement DocumentWindow (seems
too restrictive).
I think I like 2 best. Ideally the XHR spec should define what
properties the Document it returns should have.
I'd be ok with 1 as well, though it might be tricky to define "presented".
/ Jonas
