A very well compiled
thread at WebproWorld discusses what W3C is and why it is so important to conform to its standards.
W3C refers to the World Wide Web Consortium working to develop open standards for the World Wide Web (abbreviated WWW or W3) so that Web documents can be consistently displayed across all platforms.
Founded in 1994 the consortium is still the main web standard against which all websites are evaluated. The most well known W3C tool is
Markup Validation Service that checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. Other valuable tools include
RSS/Atom feed validator or
CSS validator,
Mobile content checker, and
broken link checker.
(more…) 
What is W3C? Are W3C Errors Relevant to SEO? 
