Google has an "accessible" version they are working on.
http://labs.google.com/accessible/
Accessibility does not require valid code under the newer
Web Content Accessibility Guidelines 2.0 but is required for double A compliancy under the older
Web Content Accessibility Guidelines 1.0. Not sure if it is required under US
Section 508.
Google is after content and
valid code has no relevance, however that’s not to say my mark-up is not to highest level possible and everyone should be aiming for the latest HTML 4.01 Strict.
Testing should be first. For example if you do create a valid HTML 4.01 or the slower (X)HTML Strict you may have older browsers crashing when there is no language attribute for JavaScript tag (that is not supported under the new strict standards).
There for Invalid code is ok as long as it can be justified.
I am sure Google “Test” there work. Much of Googles advances will never meet Accessibility guiltiness such as Google Maps and AdSense because it is impossible to do so.