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 service by W3C that helps check the validity of Web documents. Most Web documents are written using markup languages, such as HTML or XHTML.

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

Check that you are using a proper syntax for your comments, e.g: <!-- comment here -->. This error may appear if you forget the last "--" to close one comment, and later open another.

Test for warning output from HTML5 parser validate - with v.w.o - view: sample HTML5 document using a construct that should pass checking, with a warning output.

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.

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 …

This service uses the Feed validator software. Please report any issues on GitHub.

For elements intended to be rendered as HTML, like RSS description, make sure that all plain text content is escaped first as HTML and then again as XML. For Atom text constructs, you can …