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).
Checking conformance to W3C’s HTML 5.1 and to WHATWG’s HTML is not required, but would be nice to have in addition.
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://validator.w3.org/ redirects to https://validator.w3.org/nu/ for HTML5
https://validator.w3.org/nu/ (which is referenced by the HTML5 spec) seems to be for W3C’s draft of HTML 5.1
https://html5.validator.nu/ (which is referenced by https://validator.whatwg.org/) seems to be for WHATWG’s HTML
https://validator.nu/ seems to use the same rules as https://html5.validator.nu/
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
pictureelement (so the checker must report an error ifpictureis used). -- As also explained in the question (first bullet point), validator.w3.org redirects to the Nu validator if you want to check HTML5.