All you WCAG nuts this is the place for you: how to make Drupal more accessible. Hints, tips, discussion and patch proposals.
There is set up a small testing site for themes at http://theme.drupaler.net/.
I've tried to install all available themes (4.7.2) and categorized them according to their validatation against "Markup", "CSS" and "WCAG".
All Installed themes can be easily tested from a "Test [$theme]" link. If you are registered you are able to vote for a theme.
If a theme is missing, you should post a story with the [$theme].tar.gz
As I've started "W3C compliant CSS"-issue (http://drupal.org/node/70296) as a wish, it was the first one in the chain of validitaion.
The main critism of warning-free css-validation is the "inherit" argument, which changes nothing visible, but is needed by W3C-validation (warning free).
In my opinion there is a difference between an "explicit defined default" and an "undefined default", cause the "explicit defined" says: Yes, I'm sure, that I want to do this.
As we are experienced webusers, we know that the warning of "undefined color" doesn't make visible sense. But it's one of my most-hated questions customers asks me twice a month: "Why does your site so much validation warnings, although you are working active for accessibility?".
I'd be interested in a compiled document "bullet-pointing" status on Drupal accessibilities. What standards does it adhere to - how much? What remains to be done to make Drupal and whatever theme fully accessible according to this and that standard?
Would that be a good starting point?
Does it exist yet?