“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

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This interface to HTML5 document checking is obsolete.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.