The Firebug extension[1] will validate your page including ajax elements. Proof of this can be seen by visiting Gmail with this extension on -- the error counter just keeps ticking over!
James Bennett's note still applies -- On 5/5/06, James Bennett <[EMAIL PROTECTED]> wrote:
This actually isn't a reliable technique, because the "generated source" is not equivalent to the actual HTML Firefox received; instead, it's a serialization of the error-corrected DOM Firefox used internally to render the page.
[1] https://addons.mozilla.org/firefox/1843/ - Andrew
