[css-nesting] Use "quiet" rules for use-counting bare declarations #43973
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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] w3c/csswg-drafts#8738 [2] w3c/csswg-drafts#9492 [3] https://chromestatus.com/metrics/feature/timeline/popularity/4709 Bug: 1517290 Change-Id: Ieef0b47d85ffb5a5e58424457422686e74ea0668
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Jan 12, 2024
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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] w3c/csswg-drafts#8738
[2] w3c/csswg-drafts#9492
[3] https://chromestatus.com/metrics/feature/timeline/popularity/4709
Bug: 1517290
Change-Id: Ieef0b47d85ffb5a5e58424457422686e74ea0668
Reviewed-on: https://chromium-review.googlesource.com/5177646
WPT-Export-Revision: f0c8d351a2483a1b93d9806afada5303b667c6cd