It should be possible to transition a custom property, but right now it's not.
<rdar://problem/103404742>
Pull request: https://github.com/WebKit/WebKit/pull/7905
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/37599
Committed 258134@main (56d4a17a7f6b): <https://commits.webkit.org/258134@main> Reviewed commits have been landed. Closing PR #7905 and removing active labels.