The Markup Validation Service is a validator
by the World Wide Web Consortium
(W3C) that allows Internet
users to check pre-HTML5 HTML
documents for well-formed
markup against a document type definition
. Markup validation is an important step towards ensuring the technical quality of web pages. However, it is not a complete measure of web standards
Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on search engine optimization
has removed the use of DTD in favor of a "Living Standard", the traditional Markup Validation Service is not applicable to these formats. Validation is instead performed using an open-source "Nu Validator", an instance of which is provided by W3C.
The Markup Validation Service began as ''The Kinder, Gentler HTML Validator'', a project by Gerald Oskoboiny.
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.
In September 1997, Oskoboiny began working for the W3C, and on December 18, 1997, the W3C announced 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 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
Mark-up validators cannot see the "big picture" on a web page, but they excel at picking up missed closing tags and other technicalities.
[Castro, Elisabeth: ''HTML, XHTML & CSS, Sixth Edition'', page 345–346. Visual Quickstart Guides, Peachpit Press, 2007. ]
DTD-based validators are also limited in their ability to check attribute values according to many specification documents. For example, using an HTML 4.01 DOCTYPE,
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 intended in all browsers, even in the absence of validation errors and successful display in other browsers. The only way to ensure that pages always display as intended is to test them in all browsers expected to render them correctly.
While the W3C and other HTML and XHTML validators will assess pages coded in those 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 current CSS standards to referenced CSS documents.
* HTML Tidy
, an offline markup validation program developed by Dave Raggett
* CSE HTML Validator
, an offline HTML
* World Wide Web Consortium
The W3C Markup Validation ServiceThe W3C CSS Validation ServiceThe W3C Monitoring Service
Category:World Wide Web Consortium