Keep old behavior behind a pref.
I've kept the tests that failed enabling the pref because they're
somewhat hard to rewrite and I'd rather avoid that until we know this
will stick, specially since they test the guts of the recursive function
that I'm basically ~disabling.
I've checked that the rendering on those is sensible / matches other
browsers.
Differential Revision: https://phabricator.services.mozilla.com/D130717
