About The W3C Markup Validation Service

The W3C Markup Validation Service checks HTML documents for conformance to W3C HTML and XHTML Recommendations and other HTML standards. Related resources include:

Other resources

Documentation & Specifications

Online Tools & Other Validators

The WDG HTML validator is another excellent online validation service.
A Real Validator is a shareware HTML syntax checker for Windows systems, from the author of the WDG validator.
Site Valet by Nick Kew is a comprehensive set of Quality Assurance tools for checking and monitoring your web sites.
Clean up your HTML using HTML tidy
Validate your CSS using the W3C CSS Validation Service.
Validate your RDF using the W3C RDF Validation Service.
Validate your XML Schema documents using the W3C XML Schema Validator.

Credits

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:

A derivative version of James Clark's excellent SGML (and XML) parser SP. The version in use for this service is the "OpenSP" version from the OpenJade team. For some time it also made use of Liam Quinn's modified version, lq-nsgmls.
The LWP Perl Modules by Gisle Aas and Martijn Koster for retrieving documents.
Documentation and Error Explanations originally written by Scott Bigham.
Open source software including Apache, Perl, and Debian GNU/Linux.
Patches and ideas from: