Intent to prototype and ship: CSS content property alt text

[Emilio Cobos Álvarez]

Summary: Allow the content property to parse an alternative text value, and let screen readers account for it. Note that the spec says we should support counters, but no browser implements this, see https://github.com/w3c/csswg-drafts/issues/10387.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1899723

Pref: layout.css.content.alt-text.enabled

Specification: https://drafts.csswg.org/css-content/#alt

Standards Body: CSSWG

DevTools bug: N/A

Other browsers:

 * Blink: Shipping the single value syntax, about to ship multi-value syntax.

 * WebKit: Shipping multi-value syntax.

web-platform-tests: Parsing tests in css/css-content, accname/name/comp_name_from_content.html tests the a11y stuff (there are other internal tests for that too).

Let me know if there are any concerns here.

Cheers,

 -- Emilio