The W3C Markup Validation Service developed by the World Wide Web Consortium (W3C) allows website developers to check HTML and XHTML documents for well-formed markup and ensures the technical quality of web pages; however, is not a complete measure of web standards conformance.
The W3C Markup Validation Service started as The Kinder, Gentler HTML Validator, a project of Gerald Oskoboiny. It was later developed to be a more intuitive version of the first online HTML validator by Dan Connolly and Mark Gaither and was announced on July 13, 1994. Then in September 1997, Oskoboiny started working for the W3C, and on December 18, 1997, the W3C announced its W3C HTML Validator based upon his works.
W3C validation is important for browser compatibility and site usability, but it has not been confirmed what effect it has on search engine optimization.
Apart from validation tools for web technologies like HTML/XHTML, W3C also offers validation tools for CSS, XML schemas, and MathML.