The old code was collapsing background-position-x/y into background-position. It is not needed anymore, as returning true in canUseShorthandForLonghand for background-position has the same effect. Test: fast/css/background-position-serialize.html
<rdar://problem/102306296>
Pull request: https://github.com/WebKit/WebKit/pull/6458
Committed 256643@main (fd7fe182b10d): <https://commits.webkit.org/256643@main> Reviewed commits have been landed. Closing PR #6458 and removing active labels.