## About The W3C Markup Validation Service

### Table of contents

### About this service

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.

This validator is also An HTML validating system conforming to International Standard ISO/IEC 15445—HyperText Markup Language, and International Standard ISO 8879—Standard Generalized Markup Language (SGML) – which basically means that in addition to W3C recommendations, it can validate according to these ISO standards.

Related resources include:

- The Documentation for the Markup validation contains a lot of information on usage, installation, and development. Notably, the Help and FAQ document contains a lot of information, and is a recommended reading.
- The Source code availability information page .
- The development roadmap for this service.
- How to provide feedback on The W3C Markup Validation Service.

### Other resources

#### Documentation & Specifications

- W3C's HTML home page.
- The HTML 4.01 specification.
- XHTML 1.0: The Extensible HyperText Markup Language.
- XHTML 1.1: Module-based XHTML.

#### Online Tools & Other Validators

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:

- HTML tidy, originally developed at W3C, is a program that can help automatically clean up HTML pages.
- Validome offers a very reliable validator for HTML, XHTML and WML, in different languages.
- 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.

The W3C also hosts a number of other Open Source software projects.

### 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:

- Perl and many excellent open source Perl modules (see list in installation documentation)
- 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.
- Documentation and Error Explanations originally written by Scott Bigham.
- Interface and Design refinements by Valerio Proietti (and powered by Mootools)

- Patches, Documentation and Ideas from: Aaron Swartz, Björn Höhrmann, Brett Bieber, Chris Lilley, Christian Smith, Christoph Päper, Dan Connolly, David Dorward, David Tibbe, Etienne Miret, Frank Ellermann, Hugo Haas, Henri Sivonen, Ian Hickson, Jim Ley, Jukka Korpela Karl Dubost, Liam Quinn, Martin Dürst, Moto Ishizawa, Nick Kew, Olivier Thereaux, Patrick H. Lauke, 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.