David Dorward wrote:

On 12 Feb 2018, at 14:26, Philip Taylor (RHUoL) wrote:

No errors here :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
["http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"](<http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd>)>

Errors triggered by specific values for attributes are a good indicator that the document is being validated in HTML 5 mode and not with the DTD validator engine.

As far as I was aware, David, there is no W3C tool for validation in HTML 5 mode. Throw an HTML 5 document at http://validator.w3.org/ and you will be told :

Nu Html Checker

This tool is an ongoing experiment in better HTML checking, and its behavior remains subject to change

It doesn't claim to validate (qua validate), merely to check. Thus I always assume when someone reports a problem with HTML validation that they are talking about validation, not merely checking using an unstable tool the behaviour of which "remains subject to change".

Philip Taylor
Philip Taylor