Markup Validation Service
http://validator.w3.org/

This validator checks 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. As an alternative you can also try their non-DTD-based validator. The W3C validators are hosted on server technology donated by HP, and supported by community donations. Donate and help us build better tools for a better web. This will be added to World Wide Web Reference Subject Tracer™. This will be added to Entrepreneurial Resources Subject Tracer™. This will be added to the tools section of Research Resources Subject Tracer™.