As a first step towards support for interpolating custom properties (see bug 249293), we can start with basic support for interpolating <length> custom properties.
Pull request: https://github.com/WebKit/WebKit/pull/7615
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/37506
Committed 257906@main (bcd8cc0c0c83): <https://commits.webkit.org/257906@main> Reviewed commits have been landed. Closing PR #7615 and removing active labels.
<rdar://problem/103389360>