EPUB is already XHTML and supporting files zipped (iirc the packaging) into one file. It's very widely supported. I wonder why they don't start with it or even mention it?

EPUB lacks annotation, which is not at all trivial; I don't know how well it handles precise layout and pagination, and long-term preservation (will I be able to read it in 20 or 50 or 100 years?). But, again, I wonder why they don't improve on EPUB or at least use it as a starting point, and not seem to reinvent the wheel? (Sometimes, there's a good reason.)