The Markup Validator is a free service by W3C that helps check the validity of Web documents.
Most Web documents are written using markup languages, such as HTML or XHTML. These languages are defined by technical specifications, which usually include a machine-readable formal grammar (and vocabulary). The act of checking a document against these constraints is called validation, and this is what the Markup Validator does.
Validating Web documents is an important step which can dramatically help improving and ensuring their quality, and it can save a lot of time and money (read more on why validating matters). Validation is, however, neither a full quality check, nor is it strictly equivalent to checking for conformance to the specification.
This validator can process documents written in most markup languages. Supported document types include the HTML (through HTML 4.01) and XHTML (1.0 and 1.1) family, MathML, SMIL and SVG (1.0 and 1.1, including the mobile profiles). The Markup Validator can also validate Web documents written with an SGML or XML DTD, provided they use a proper document type declaration.
Related resources include:
In addition to this validator, the W3C is offering a number of other tools to help you check other types of documents (CSS, RDF, P3P, ...), find broken links in your Web pages, and so on. All these tools are listed on the W3C's QA Toolbox.
There are also many excellent tools developed outside W3C to help improve the quality of Web pages:
The W3C also hosts a number of other Open Source software projects.
The first online HTML validation service was created by Dan Connolly and Mark Gaither.
The W3C Markup Validation Service was created and maintained by Gerald Oskoboiny. In a previous incarnation it was known as "The Kinder, Gentler, HTML Validator" ("Kinder, Gentler" than Dan and Mark's original), but has since found a new home at W3C, and is now maintained under the auspices of the Quality Assurance Activity.
This service uses:
Patches, Documentation and Ideas from: Aaron Swartz, Björn Höhrmann, Brett Bieber, Chris Lilley, Christian Smith, Christoph Päper, Dan Connolly, David Dorward, David Tibbe, Hugo Haas, Ian Hickson, Jim Ley, Jukka Korpela Karl Dubost, Liam Quinn, Martin Dürst, Nick Kew, Olivier Thereaux, Roland W. Crowl, Scott Bigham, Sierk Bornemann, Steph Troeth, Sean B. Palmer, Shane McCarron, Terje Bless, Ville Skyttä, ...and the great user community on www-validator@w3.org.