Repair of poorly constructed page is extra-credit. I don't think that we should be espousing theories about what the UA should provide if why don't apply to actual content in the world. We need evidence that something is effective, not only readily achievable. If you just use HTML, you don't get a usable product. You have people using tables and frames to implement the same functional object.
People have N ways of building a header bar.
The specs don't tell you how to do this. You have evidence of user agents doing good things in structured navigation, but you have not criteria for drawing the line. More research would be necessary to know what's a minimum requirement for a markup language. I think we can make the argument about a lot of checkpoints. Then we'll continue to get the complaint that people don't know when they conform.