W3C Markup Validation Service
The Markup Validation Service is a validator by the World Wide Web Consortium (W3C) that allows Internet users to check HTML and XHTML documents forwell-formed markup. Markup validation is an important step towards ensuring the technical quality of web pages; however, is not a complete measure of web standards conformance. Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on search engine optimization.
While the W3C and other HTML and XHTML validators will assess pages coded in those formats, a separate validator like the W3C CSS validator is needed to confirm that there are no errors in the associated Cascading Style Sheet. CSS validators work in a similar manner to HTML and XHTML validators in that they apply current CSS standards to referenced CSS documents.