I’m looking for a hosted Web app that validates HTML5 documents.
The tool must be able to check the document for conformance to the HTML5 W3C Recommendation from 2014-10-28. That means, for example, that it should report an error for a document that uses a
picture element (as it’s not part of W3C’s HTML5 Rec).
Pasting the document is preferred, but checking via URL or by uploading a file would be okay, too.
Support for extensions (like Microdata, RDFa, …) would be great, but it’s not required.
What I’ve tried:
https://www.onlinewebcheck.com/ (which is referenced by https://validator.whatwg.org/) seems to use HTML 5.1 (at least, when validating a document with the
pictureelement, their warning speaks of "an experimental HTML 5.1 element")
https://checker.html5.org/ seems to be for WHATWG’s HTML