Error messages link to relevant info

Hello, firstly thanks for the validator it's so useful!
I have a question/suggestion, validation errors don't have any further
information, e.g. a link to the relevant item in the spec.
For example, I'm getting this error... "Error: An element with the
attribute tabindex must not appear as a descendant of the a element."
But there is no link to the rule of the HTML spec this breaches. I want to
know exactly what interactive elements are allowed within the A tag, but I
can't find where information is in the spec, it would be really helpful if
errors on the validation page linked to the relevant articles.

Kind Regards.

PS, if you know where the relevant article is relating to the above error,
please send me a link, I still haven't found it!