Houyhnhnm Land is one-hundred-percent valid XHTML 1.0 Transitional.
I don't, I must confess, know entirely what that means; I already knew the site was fairly accessible, since I checked in several browsers, including a text-based one. Validation doesn't guarantee that your site will be accessible; and not being valid will not necessarily interfere with the accessibility of your site. But someone somewhere said these are the things that should be done.
I have accomplished it. I feel validated.