Most, if not all PRO designs have been built to meet W3C compliancy. In order for your site to remain W3C compliant, you have to use proper coding throughout your site. There are a couple tools out there that you can load into your browser that will help you monitor the compliancy of your site. A favorite is HTML Tidy – it is available as an add-on for the Firefox browser at http://www.mozilla.com/firefox/, or at http://www.w3.org/People/Raggett/tidy/