Implement at-rule for use with @supports.
It's behind a flag for now. Note that there is a CSSWG resolution
(w3c/csswg-drafts#2463 (comment))
but no spec. We don't support the proposed but not yet ratified
extension of having a full block
(w3c/csswg-drafts#6966)
Bug: 40211832
Change-Id: Ic80e28abe9e9fce6d44b1a52311aba7b60a8caa2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5972643
Reviewed-by: Anders Hartvoll Ruud andruud@chromium.org
Commit-Queue: Steinar H Gunderson sesse@chromium.org
Cr-Commit-Position: refs/heads/main@{#1376258}