[selectors-4] HTML and case-sensitivity

[annevk]

HTML has whatwg/html#3372 (in general) and whatwg/html#2738 (attr()-specific).

I'm not entirely sure what can be done here given the constraints, but I suppose at least https://drafts.csswg.org/selectors/#case-sensitive and https://drafts.csswg.org/selectors/#attribute-case could be updated to be more accurate about HTML.

(I also think it's a little misleading to talk about HTML and XML, since you can use XML elements inside HTML through scripting, but the HTML rules apply since it's an HTML document (https://dom.spec.whatwg.org/#concept-document-type).)

[SelenIT]

Issue #2101 also is probably related.