Navigation First Still OK?
One the earlier accessibility mantras was to put content before navigation. The theory was that that screen reader users found reading the same main menu options on each page to be distracting and time-consuming.
Yet visual users generally prefer navigation menus to the left (which means they usually come first in the code). Interestingly readers of right-to-left scripts like Hebrew and Arabic want their navigation to be to the RIGHT (where their eyes first hit when reading).
In any event, some experts advocating placing navigation on the right so that content would come first in screen readers. Another variation was to put navigation after content in the HTML, but use CSS to move the navigation in the top/left where visual users were expecting. The final solution was to keep navigation first, but implement a "Skip Main Menu" strategy. Ironically, I found a Usability Source Order Survey in Australia which supports the "Skip Main Menu strategy" as the best choice.
According to this survey, it's not just visual users who expect navigation, but many screen reader users as well. What many users really wanted a quick bypass once they got familiar with it, but they were still expecting it to be there first.
I would compare to previews on a DVD. I expect the previews to come before the main feature movie, but am grateful when I can skip them (especially if the DVD is due back at the store before midnight). On the other hand, if the previews come AFTER the movie credits, I will be a bit confused (especially if I'm expecting the DVD bonus interview features next).
This is kind of a relief from a developer point of view because you can really use the same code for everyone with an extra tweak (actually, a lot of people would like "Skip Main Menu" - including keyboard users). Many visual users do prefer left hand or top navigation over right and bottom, so using right/bottom navigation seems awkward. I'm also glad to be avoiding CSS floating hacks which move content out of their source order. I've always thought this was dangerous, because other users (e.g. low vision users or color blind users) use alternate CSS sheets. The more you keep the content flow the same across alternate styles, the more accessible you are.