-apple-has-attachment seems like the logical choice given it's only exposed to the Apple apps.
It also should not parse when <attachment> isn't enabled.
(In reply to Tim Nguyen (:ntim) from comment #1) > It also should not parse when <attachment> isn't enabled. That part is already the case: https://searchfox.org/wubkat/rev/0d695812f5b0daef531a9760903457364d2265ff/Source/WebCore/css/parser/CSSSelectorParser.cpp#783-786
<rdar://problem/120599170>
Pull request: https://github.com/WebKit/WebKit/pull/22478
Committed 272820@main (ee0b8ecdb38d): <https://commits.webkit.org/272820@main> Reviewed commits have been landed. Closing PR #22478 and removing active labels.
Re-opening for pull request https://github.com/WebKit/WebKit/pull/22544
Committed 272821@main (4f86be215342): <https://commits.webkit.org/272821@main> Reviewed commits have been landed. Closing PR #22544 and removing active labels.