Website Markup Validation

Use this site to check if your website is standards compliant. While it is debated how important it is to pass, it is still useful for cleaning up the code on your site. The site is described as checking "the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK content, or to find broken links, there are other validators and tools available."