commit 7700322c0107f030aafeeb4704efc5e2b4a15fca
author Anders Hartvoll Ruud <andruud@chromium.org> Thu Jan 25 10:23:20 2024
committer Chromium LUCI CQ <chromium-scoped@luci-project-accounts.iam.gserviceaccount.com> Thu Jan 25 10:23:20 2024
tree c9115be671da96d1addc3dd90b915194158d26d0
parent bca23ef1dfccb25ace64d5afd6307645108e1a8f

[css-nesting] Add signaling rules to aid use-counting

The CSSWG wants to change a few things about how "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]. The parse-time expansion part also affects
our existing implicit wrappers generated by bare declarations within
nested grouping rules (e.g. nested @media). Those wrappers rules may
get their specificity lowered.

Actually making these changes is easier said than done, since we now
have to worry about the compat risk. A naive use-counter added
previously [3] 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. Also, it doesn't cover
the specificity change for the (existing) implicit wrapper rule.

This CL is the first in a chain which (hopefully) gives us
the tools needed to use-count for these changes.

The "signaling rules" added by this CL, are simply normal style rules
which carry a "signal" that is converted to a use-count (or not)
later (see CSSSelector::Signal for more detail). This alone is not
enough to cover the cases we care about for [1] and [2], so for now
this new capability is not used outside of tests.

[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: Iad193ff1da07039b9876fadd413f5c9654be08ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5201402
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1251993}