As per CSSWG resolution: https://github.com/w3c/csswg-drafts/issues/3673

Some details are not clear, so implement it behind a pref, disabled by
default.

Differential Revision: https://phabricator.services.mozilla.com/D157641

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1791375
gecko-commit: 05ef9b63051cd64e8e8ef8bd592af7d1dcfae146
gecko-reviewers: emilio