Intent to prototype and ship: CSS nested declarations rule

[Emilio Cobos Álvarez]

Summary: This fixes rather confusing CSS nesting behavior, see [1] and
all related discussions.

Bug: https://bugzil.la/1918408

Standard: https://drafts.csswg.org/css-nesting-1/#nested-declarations-rule

Standards Body: CSSWG (W3C)

Platforms: All

Preference: No, it was rather invasive to the parser and I think the
real risk of breakage is not too big.

DevTools bug: Some work going on in bug 1919853, some more to do.

Links to standards-positions:

* https://github.com/mozilla/standards-positions/issues/1048

Other browsers:

* Blink has this enabled by default (to be shipped to release still).
* WebKit is supportive of this
(https://github.com/WebKit/standards-positions/issues/369)

WPT: This is part now of interop 2024, and there are existing tests in
css/css-nesting/.

Cheers,

-- Emilio

[1]: https://github.com/w3c/csswg-drafts/issues/8738