style: Tweak background: transparent handling so that color: transparent doesn't override UA sheet backgrounds.
style: Tweak background: transparent handling so that color: transparent doesn't override UA sheet backgrounds.

Differential Revision: https://phabricator.services.mozilla.com/D68408

KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo