Initial feedback from annevk

Spoke to @annevk some of his initial comments need fixing:
- [ ] I had a quite look, not quite sure it should be modeled on top of HTML, but maybe that's good enough
- [ ] I also missed some kind of model description, which objects keeps state around etc.
- [ ] And there's some stuff around images that's going to be more complicated. Basically and background-image et al share some kind of code path. So HTML needs to be abstracted a bit to have some concept of "image fetch" and CSS should probably use that. _Might also be worth looking at open issues against HTML_
- [ ] CORS-same-origin is part of HTML but could be uplifted to Fetch

_zcorpan / Simon Pieters is probably a good person to work with. I believe Tab Atkins also plans on looking at this_