You can validate your site with the W3C CSS Validator using the following url.
http://jigsaw.w3.org/css-validator/validator?uri=http://yoursite.com&profile=css3
Or by adding following code in your page:
<a href="http://jigsaw.w3.org/css-validator/validator?uri=http://<?php echo $_SERVER["HTTP_HOST"] ?><?php echo $_SERVER["REQUEST_URI"] ?>&profile=css3″>Valid CSS</a>
