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, …

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.

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

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 …

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.

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 …

Validation is one of the simplest ways to check whether a page is built in accordance with Web standards, and provides one of the most reliable guarantee that future Web platforms will …

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 …

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

This is the W3C Feed Validation Service, a free service that checks the syntax of Atom or RSS feeds. The Markup Validation Service is also available if you wish to validate regular Web pages.