We added basic support for interpolating custom properties in bug 249312, but it only works with explicit keyframes and no implicit keyframes.
Pull request: https://github.com/WebKit/WebKit/pull/7679
Committed 257911@main (c3591d297f09): <https://commits.webkit.org/257911@main> Reviewed commits have been landed. Closing PR #7679 and removing active labels.
<rdar://problem/103397173>