The issue in Web accessibility is the fact that blind and visually-impaired people need the single biggest boost to achieve equivalence, since the real-world Web is a visual medium.
Navigation is important, but the single biggest issue is prodding Web developers into learning how to make standards-compliant sites. They've quite simply been doing it wrong all along.
Even if you set aside the need for valid code, it is ridiculously easy to find non-government sites that flunk even the simplest and most canonical requirements of the Web Content Accessibility Guidelines, like using alt texts for images.
In any event, accessibility is almost as poorly-known now as it was 2.5 years ago when I started work on my book. That's because most 'Web' developers aren't making Web sites at all, since they don't have a clue what valid HTML and CSS means.