The Markup Validation Service is a validator by the World Wide Web Consortium (W3C) that allows Internet users to check HTML and XHTML documents for well-formed markup. Markup validation is an important step towards ensuring the technical quality of web pages. However, it is not a complete measure of web standard conformance.  Though W3C validation is significant for browser compatibility and website usability, It has not-been confirmed what effect It has one search engine optimization .
The Markup Validation Service began as The Kinder, Gentler HTML Validator , a project by Gerald Oskoboiny.  Dan Connolly and Mark Gaither, which was announced on July 13, 1994. 
In September 1997, Oskoboiny began working for the W3C, and on December 18, 1997, the W3C published its W3C HTML Validator based upon his works.  In November 2008, the W3C released the validator.nu HTML5 engine and the ability to check documents for conformance to HTML5 
W3C also offers validation tools for web technologies other than HTML / XHTML, such as CSS ,  XML schemas , and MathML .
Many major web browsers are often tolerant of certain types of error, and may well be a syntactically correct document. Some other XML documents may also be validated if they refer to an internal or external DTD .
Mark-up validators can not see the “big picture” on a web page,   but they excel at picking up missed tags and other technicalities.
DTD-based validators are also limited in their ability to check. For example, using an HTML 4.01 DOCTYPE, bgcolor = “fffff” is accepted as valid for the “body” element even though the value “fffff” is missing a preceding ‘#’ character and contains only five (instead of six) hexadecimal digits . Also, for the “img” element, width = “really wide” is also accepted as valid. DTD-based validators are technically not able to test for these types of attribute value problems.
Pages may not display as expected in all browsers, even in the absence of validation errors and successful display in other browsers. The only way to ensure that they are expected to be in their way. 
While the W3C and other HTML and XHTML validators will assess pages in these formats, a separate validator like the W3C CSS validator can check that there are no errors in the associated Cascading Style Sheet . CSS validators apply the CSS standard to referenced CSS documents. 
- HTML Tidy , an offline markup validation program developed by Dave Raggett of W3C
- CSE HTML Validator , an offline HTML and CSS validator
- World Wide Web Consortium (W3C)
- ^ Jump up to:a b “About the W3C Markup Validation Service” . W3C Markup Validation Service . World Wide Web Consortium . Retrieved 2011-05-20 .
- Jump up^ Connolly, Dan (1994-07-13). “ANNOUNCE: HaL Interactive HTML Validation Service” . www-html (Mailing list) . Retrieved 2008-06-28 .
- Jump up^ Oskoboiny, Gerald (2003-03-22). “History of the Kinder, Gentler HTML Validator” . Retrieved 2008-06-28 .
- Jump up^ “What’s New at The W3C Markup Validation Service” . validator.w3.org . Retrieved 2016-05-31 .
- ^ Jump up to:a b “About the CSS Validator” . World Wide Web Consortium . Retrieved 2012-06-24 .
- ^ Jump up to:a b c Castro, Elisabeth: HTML, XHTML & CSS, Sixth Edition , page 345-346. Visual Quickstart Guides, Peachpit Press, 2007. ISBN 0-321-43084-0