Remove redundant UA CSS requirement

[scottaohara]

closes #2521

removes the following from the spec:

If a CSS selector includes a WAI-ARIA attribute (e.g.,
input[aria-invalid="true"]
), user agents MUST update the visual display of any elements matching (or no longer matching) the selector any time the attribute is added/changed/removed in the DOM.

as CSS already defines how UAs are to handle element/attribute selectors

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[scottaohara]

i know the issue is marked for discussion in an upcoming ARIA editor's meeting. We can discuss this PR / what may need to change about it then as well. cc @pkra

[pkra]

If we remove this, we should also remove the normative statement in 8.7.

I think it would be good to keep a note on the general topic anyway, with a link to somewhere in CSS where these normative statements are made.

I put it on the editors' agenda because I've been wondering about what "supporting host languages" means these days, i.e. what does ARIA expect from a "supporting host language"?

[pkra]

From our conversation: I'll update the PR to rewrite the second occurrence to make a reference https://drafts.csswg.org/selectors/#attribute-selectors (via xref https://respec.org/xref/?term=attribute+selector).