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, …

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.

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

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 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.

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

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 …

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 …

For programmatic checking of modern HTML documents, use the API provided by the W3C HTML Checker. To do that, call https://validator.w3.org/nu/ or the URL for another instance of the …

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 …