As a web site programmer, the Firefox HTML checker goes a long way to helping me write (almost) error free W3C compliant code. However, earlier versions of the checker could only validate basic pages so if you use AJAX extensively, it wasn't a lot of help.
The new version now has a facility to check the revised page coding after a JavaScript/AJAX call. BRILLIANT! There are a few bugs in the checker but it's a huge leap forward. If you don't use this Firefox add-on, get it now.
https://addons.mozilla.org/en-US/firefox/addon/249
So if Firefox can do this, when will the search engines finally get with the program (pun intended) and implement their own Javascript followers so they can index AJAX generated content?