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 …

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.

W3C's easy-to-use markup validation service, based on SGML and XML parsers.

W3C provides various free validation services that help check the conformance of Web sites against open standards. You are most likely here because this address appeared in logs for your website.

It compares your HTML document to the defined syntax of HTML and reports any discrepancies. Learn more about the Markup Validator and the languages it can validate.

W3C's easy-to-use markup validation service, based on SGML and XML parsers.

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 W3C …

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 …

First, check our Help and FAQ document, as well as the Web authoring FAQ to see if your question has been answered there. The two most common problems are: Validating pages with ampersands (&'s) …

0.8.4 may sound like a very minor step from the version 0.8.3 released in August, but this new release of the W3C Markup Validator brings some very important change: in addition to checking documents …