We should support at least appearance auto and none, which are aliases that override -webkit-appearance.
FYI: Chrome is going to ship 'appearance' property soon.
FYI https://bugs.chromium.org/p/chromium/issues/detail?id=963551 is now fixed and looks like it's targeting M84 (reaching stable mid-July per https://www.chromestatus.com/features/schedule )
I think trying to coordinate on the timing on this can reduce compat problems, and allow for clearer messaging to web developers to start using unprefixed appearance.
Issue for Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1620467