Fix background-valid.html.
This aligns with our behavior and Chrome's. Colors generally serialize
to their resolved form in getComputedStyle(), and we only preserve
keyword colors during parsing for legacy reasons (but this is not the
case, because the transparent color comes from the engine).
Differential Revision: https://phabricator.services.mozilla.com/D224760
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1917052
gecko-commit: f84e7920a7cda35d340b4be1446b28674a6518de
gecko-reviewers: dholbert