Itiel spotted this, but the bug is:

button:not([disabled], [selected]) {
  color: blue;
}

displays as:

button:not([disabled][selected]) {
  color: blue;
}

in the inspector.

Comment on attachment 9195976 [details]
Bug 1685621 - Fix serialization of complex :not(). r=#style

Beta/Release Uplift Approval Request

• User impact if declined:

  • Broken/misleading output in DevTools rules view when inspecting certain :not() selectors
  • Broken output when returning CSS selectors via JS (not sure how frequent that is though)

• Is this code covered by automated tests?: Yes

• Has the fix been verified in Nightly?: No

• Needs manual test from QE?: No

• If yes, steps to reproduce:

• List of other uplifts needed: None

• Risk to taking this patch: Low

• Why is the change risky/not risky? (and alternatives if risky): Makes :not() use the same codepath as :is() and :where() which shipped in Firefox 77. This has tests as well.

• String changes made/needed: no

Backed out for perma failures.

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&revision=899944ef4aa72a26a3de63d48f952213ec73b2b8&selectedTaskRun=TJNsSImrT6ykbIKqqurEvA.0

Log: https://treeherder.mozilla.org/logviewer?job_id=326154715&repo=autoland&lineNumber=6269

Backout: https://hg.mozilla.org/integration/autoland/rev/2405ffdc136db3563bc49962aeb4f709c7a8e083

https://hg.mozilla.org/mozilla-central/rev/7e9efe4d1265
https://hg.mozilla.org/mozilla-central/rev/84e497a940ab

Comment on attachment 9195976 [details]
Bug 1685621 - Fix serialization of complex :not(). r=#style

approved for 85.0b8

https://hg.mozilla.org/releases/mozilla-beta/rev/7b25c53d9b58
https://hg.mozilla.org/releases/mozilla-beta/rev/f71eaa9431a6