“This interface to HTML5 document checking is deprecated”

Question

While doing a project I was checking it for W3C errors (using html validator plug-in for Firefox), and in the process I found a warning that never showed up

Answer 1

All it means is that the old validator on the W3C's site is no longer able to accurately check the current spec of HTML5 that browsers use. Certain elements (like hgroup) that were in the process of being adopted were ultimately dropped, while others were altered or added.

So all it means is that that validator is outdated and thus not the best place to check your HTML5 documents.

Answer 2

Maintainer of the W3C HTML Checker here. I'm the one who added that “Warning: This interface to HTML5 document checking is deprecated” message a few weeks back.

You actually won’t see that message now, because in the mean time I’ve set up a mechanism that causes requests to be automatically redirected from the old legacy W3C Markup Validator at https://validator.w3.org/ to the current W3C HTML Checker at https://validator.w3.org/nu/

If you're curious, here's the change diff:

https://github.com/w3c/markup-validator/commit/1b12fe496bb457950944782f133f547cee3cef56

All that said, if you’re using the obsolete SOAP 1.2 API to the legacy Markup Validator (or using some tool that relies on it), you may now see a message saying:

This interface to HTML5 document checking is obsolete. Use an interface to https://validator.w3.org/nu/ instead.

If you’re seeing that message in output from some tool/library you use, then you need to report it as a bug to the developers of that tool/libary so they can update it to instead use the current API to the https://validator.w3.org/nu/ checker.