There is currently no layout support for those CSS properties, only parsing, so let's gate it behind it a separate pref so feature detection is correct.
<rdar://problem/110674890>
Pull request: https://github.com/WebKit/WebKit/pull/14897
Committed 265126@main (a40e16fe3a7e): <https://commits.webkit.org/265126@main> Reviewed commits have been landed. Closing PR #14897 and removing active labels.