coopster - 6:21 pm on Dec 22, 2008 (gmt 0)

If you attempt to validate the source code like that you are going to get errors on the <?php ?> tags. You need to let your server render the code, view source in the browser, and copy/paste the rendered code into the validator. Or just give the validator the URI and it will retrieve the rendered source for validation.

You mention that if you leave the DOCTYPE it will indeed validate, but your AJAX won't function properly ... yet you have not shown any JavaScript here at all. The likely issue is that you are using properties or methods in the JavaScript code that are not recognized by the Gecko engine.