I’ve just finished my latest AJAX project – W3C Validator.JS. It is a real (x)html markup validator, that uses the W3C API for head requests. Except my own work on that website, W3C Validator.JS also utilizes two excellent AJAX/JSON scripts created by Jacob Seidelin of Nihilogic and Simon Wilson’s JSON-HEAD.
You may also put a validation link to W3C Validator.JS directly from your website using the following URL:
<a href="http://validator.wemakesites.net/#http://www.yourwebsite.com" rel="validate" title="Validate my Markup via W3C Validator.JS">Valid Markup</a>
To illustrate this, click the following link to validate my WordPress blog (unfortunately its markup is invalid).
Have fun with W3C Validator.JS!