What is the issue with the HTML Standard?

This is originated from https://bugzilla.mozilla.org/show_bug.cgi?id=1881199.

The validation anchor can be null as the anchor parameter of setValidity(flags, message, anchor) is optional, https://html.spec.whatwg.org/multipage/custom-elements.html#dom-elementinternals-setvalidity.

But https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#interactively-validate-the-constraints says

For elements that are form-associated custom elements, user agents should use their validation anchor instead, for the purposes of these actions.

It seems like Blink fallback to FACE itself when validation anchor is null. It looks reasonable to me, but I am not sure if this is something we would like to mention in the spec.