Hi!
That’s being worked on at https://bugzilla.mozilla.org/show_bug.cgi?id=1654731
It’s a quite difficult part of the codebase, but thankfully a brave contributor who already added a preview of the class “as you type” is currently writing a patch to add this feature. Feel free to follow the progress on Bugzilla. I don’t think this will make it in time for 96, but 97 should be ok.