They already have to deal with the fact that doesn't work in the real world. Currently a PR war between two US presidential candidates about having made their sites accessible. Needs to be subject to user override. Instances where you don't need to supply along with your content any rendering information. It's not the structure of the content or the presentation.
Navigation is a separate concept.
I actually put navigation and comprehension together. I think that's too general. Navigation can be completely separate and imposed, like a site map. Probably be more so with Web phones and Palm pilots and such. Some of these things are orthogonal, not parallel.