tree 52d5d0837ac27dc217aae65b263d56150919eb23
parent 12998136aa625935c0eb8c4b6411066cea57f882
author Anders Hartvoll Ruud <andruud@chromium.org> 1705074668 -0800
committer Chromium WPT Sync <blink-w3c-test-autoroller@chromium.org> 1705074668 -0800

[css-nesting] Use "quiet" rules for use-counting bare declarations

The CSSWG wants to change how non-leading bare declarations work.
Instead of "shifting up" those declarations to the rule's main
declaration block, we want to treat them as wrapped by an &-rule,
except with parse-time expansion of selectors instead of the normal
behavior for '&' [1][2].

Actually making this change is easier said than done, since we now
have to worry about the compat risk. A naive use-counter added
previously shows that ~0.16% of page loads at some point parse
bare declarations after a nested rule. This is too high, and we
need a more advanced use-counter.

This CL adds the concept of "quiet rules", which are generated
automatically from detecting bare declarations in CSSParserImpl
and mostly treated normally until we reach StyleCascade.
In StyleCascade we cascade with and without the quiet rules,
check if quiet rules made a difference, and trigger a use-counter
based on that.

[1] https://github.com/w3c/csswg-drafts/issues/8738
[2] https://github.com/w3c/csswg-drafts/issues/9492
[3] https://chromestatus.com/metrics/feature/timeline/popularity/4709

Bug: 1517290
Change-Id: Ieef0b47d85ffb5a5e58424457422686e74ea0668