A proper browser can display all HTML documents crated like, ever, if you can live with some imperfections that are there to deal with improper browsers in the first place.
I mean, I get your point, and it makes sense, and for a similar reason we didn't get WebSQL. But I think HTML is special in this regard. Everyone just accepted the complexity of dealing with all sorts of documents.
HTML 5 is a "living standard", which is a horrible idea for documents that might be archived for 10 years, like email.
Note, I'm not condoning the status quo, but let's think about what we actually want here.