About The W3C Markup Validation Service

Table of contents

About this service

The W3C Markup Validation Service (or HTML Validator, as it used to be called) is a free (Open Source) tool that checks Web documents for validity.

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 not, however, strictly equivalent to conformance checking.

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:

• 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 TODO list for The W3C Markup Validation Service.
• How to provide feedback on The W3C Markup Validation Service.

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

• 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.

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