The W3 checks Web sites for valid HTML, XHTML, SMIL, MathML and other markup code. Just enter the URL for your Web site, click Check and the validator lists all of the markup problems it finds.
The W3 site markup validator
A quick check of my personal blog returned an error list that’s 102 items long. After catching my breath, I started digging through the errors and their accompanying descriptions and found that I probably have a single WordPress plug-in that’s causing all the errors.
Ouch. My personal blog has a few markup errors.
W3 also offers online tools to validate CSS style sheets, RSS and Atom feeds, MobileOK content, and to locate broken links. The validator tools are free to use and only require a Web browser and Internet connection.