The specifics of how this is going to work are still getting spec'd /
discussed in https://github.com/w3c/csswg-drafts/issues/6576, but this
allows DevTools to work fine and the feature to be complete enough for
Nightly experimentation (with the other in-flight patches).
Otherwise devtools crashes when trying to inspect pages that use them.
Differential Revision: https://phabricator.services.mozilla.com/D124656