See https://wpt.fyi/results/css/css-properties-values-api/var-reference-registered-properties.html?run_id=5133309367025664&run_id=5150860515999744, subtest "Empty universal custom property can be substituted with var()" has regressed. Bisecting points at 267643@main.
<rdar://problem/117690684>
Pull request: https://github.com/WebKit/WebKit/pull/19833
Committed 270053@main (48bfdb7832dc): <https://commits.webkit.org/270053@main> Reviewed commits have been landed. Closing PR #19833 and removing active labels.