After validating inputs, check parent fieldsets, too

[Boldewyn]

Apparently, :valid and :invalid should also match <fieldset>s with respective elements: https://drafts.csswg.org/selectors/#issue-4558f10d. That means we should at least maintain the hf-(in)valid classes on them.

We also have to get straight, what HTMLFieldSetElement.prototype.validity et al. is supposed to do. The W3C spec is a bit fuzzy here. Same for WHATWG. As of now we ignore it mostly due to exclusion in is_validation_candidate().