This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK …

Validate by Direct Input ... If your document is on the Web, you can validate it by entering its address instead. Home About... News Docs Help & FAQ Feedback Contribute

The Markup Validator is a free tool and service that validates markup: in other words, it checks the syntax of Web documents, written in formats such as (X)HTML.

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 …

This list conveniently gathers all the error messages currently used by the validator when processing Web documents, and the community-contributed explanations to these error messages, whenever …

If your document is on the Web, you can validate it with the same advanced options by entering its address instead.

Below is a listing of W3C's various validation services, links to the services themselves, the user-agent header being sent and how to find out more information on each.

The W3C Markup Validator provides Perl/CGI/SGML/XML/DTD-based validation of a variety of document types. SGML and DTDs are older technologies that never found wide use on the Web, so …

Changes include: More explanations for most of the validation error messages. New documentation on installing the Markup Validator locally. The "fussy" parsing mode is no longer available and will be …

This page lists a few test documents and test scenarios for the W3C Markup Validator make sure everything works fine after making changes to the validator. If you can think of interesting test cases …