W3C Markup Validation Service

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The Markup Validation Service by the World Wide Web Consortium (W3C) allows Internet users to check HTML documents for conformance to HTML or XHTML standards. It also provides a quick method for web page authors to check their posted pages for mark-up errors.[1]



The Markup Validation Service began as The Kinder, Gentler HTML Validator, a project by Gerald Oskoboiny.[2] It was developed to be a more intuitive version of the first online HTML validator written by Dan Connolly and Mark Gaither, which was announced on July 13, 1994.[3]

In September 1997, Oskoboiny began working for the W3C, and on December 18, 1997, the W3C announced its W3C HTML Validator based upon his work.[4]

Method of operation

HTML validators operate by comparing the mark-up on a web page to the W3C standards. The standards vary depending upon the declared version and so the validator will start by reading the DOCTYPE declaration to see which set of standards to apply.[1]

Once the validator has read the page and determined the applicable standards it looks for such things as missing opening or closing tags, missing quotation marks and other hand-coding errors.[1]

The validator then provides a report indicating that the coding is correct or not. Errors are noted in a list. One error, such as neglecting to close a tag, can cause a cascade of errors through the page, producing dozens or even hundreds of noted errors. However when the page author addresses the first error listed it will also eliminate the "cascade errors".[1]

Browser accommodation

Many major browsers are often tolerant of certain types of error, and may display a document successfully even if it is not syntactically correct. Certain other XML documents can also be validated if they refer to an internal or external DTD.

W3C also offers validation tools for web technologies other than HTML/XHTML, such as MathML or CSS.

Validator criticism

All mark-up validators suffer from an inability to see the "big picture" on a web page.[1] However they excel at picking up missed closing tags and other technicalities. This does not mean that the page will display as the author intended in all browsers. Even if validated, all web pages should be tested in as many different browsers as possible to ensure that the limitations of the validator are compensated for and that the page works correctly.[1]

CSS validation

While the W3C and other HTML and XHTML validators will assess pages coded in those formats, a separate 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.[1]

List of markup validation for common websites

Website or software's homepage Pass? Number of errors Number of warnings Date validated
www.google.com No 42 2 Jan 15, 2010
www.google.co.uk No 44 2 Jan 15, 2010
www.yahoo.com No 128 26 Jan 15, 2010
www.msn.com No 1 0 Jan 15, 2010
www.aol.com No 1182 584 Jan 15, 2010
www.ask.com No 129 32 Jan 15, 2010
www.amazon.com No 1184 77 Jan 15, 2010
www.amazon.co.uk No 1028 95 Jan 15, 2010
www.apple.com No 7 1 Jan 15, 2010
www.youtube.com No 212 68 Jan 15, 2010
www.facebook.com No 41 0 Jan 15, 2010
www.myspace.com No 141 80 Jan 15, 2010
www.twitter.com No 83 0 Jan 15, 2010
www.wikipedia.org No 1 2 Jan 15, 2010
www.bbc.co.uk Yes 0 0 Jan 15, 2010
www.microsoft.com No 394 35 Jan 15, 2010
www.firefox.com Yes 0 2 Jan 15, 2010
www.opera.com Yes 0 0 Jan 15, 2010
www.apple.com/safari No 12 1 Jan 15, 2010
www.google.com/chrome No 22 9 Jan 15, 2010

See also


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Castro, Elisabeth: HTML, XHTML & CSS, Sixth Edition, page 345-346. Visual Quickstart Guides, Peachpit Press, 2007. ISBN 0-321-43084-0
  2. "About the W3C Markup Validation Service". W3C Markup Validation Service. World Wide Web Consortium. http://validator.w3.org/about.html. Retrieved 2008-06-28. 
  3. Connolly, Dan (1994-07-13). "ANNOUNCE: HaL Interactive HTML Validation Service". www-html mailing list. http://lists.w3.org/Archives/Public/www-html/1994Jul/0015. Retrieved 2008-06-28. 
  4. Oskoboiny, Gerald (2003-03-22). "History of the Kinder, Gentler HTML Validator". http://impressive.net/people/gerald/1996/ugweb/validate/changes.html. Retrieved 2008-06-28. 

External links

ja:W3C Markup Validation Service pl:W3C Markup Validation Service sv:W3C Markup Validation Service zh:W3C驗証服務

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