Ok, everyone! Time to test if your site is W3C valid. Just mosey on over to http://validator.w3.org and test your site (and subpages, while you are at it) to see if your site is valid.
What is W3C validation? Well, according to WikiPedia, it is:
The Markup Validation Service by the World Wide Web Consortium (W3C) allows Internet users to check HTML documents for conformance to HTML or XHTML standards. It also provides a quick method for web page authors to check their posted pages for mark-up errors.
Should you worry if your site isn’t HTML/xHTML valid? Well, not necessarily. Using the W3C Validator test is a good way to catch errors that could allow search engines to heavily penalize your site. While validation doesn’t mean your site is necessarily “perfect” is does mean that all the major search engines will be able to crawl your website without more easily.
Content management systems like WordPress, Joomla and Expression Engine are notorious for being W3C Validation headaches for designers, developers and basic users. So, take extra care when using a template you would like to use and make sure the developer/designer made it valid.