[css-fonts-4] Moving to "swap" should fallback elements rendered as invisible

[frivoal]

Issue first reported and discussed here: tabatkins/specs#46

(part of #1357)

Agenda+ since implementations are about to ship.

[frivoal]

cc @yoavweiss

[yoavweiss]

From my perspective @tabatkins answered the questions I had and corrected my misunderstanding, so no further action is needed.