Specify the hidden attribute as a presentational hint

[zcorpan]

It seems browsers implement hidden as a presentational hint, so the spec should do that as well.

I think we should definitely change hidden to a presentational hint in a separate PR.

After looking at the text right above the hidden CSS, it says "Some rules are intended for the author-level zero-specificity presentational hints part of the CSS cascade; these are explicitly called out as presentational hints."

Does this mean that we can just add a sentence above the hidden CSS that says the hidden CSS is a presentational hint?

(Yes.)

Originally posted by @josepharhar in #7475 (comment)