I think Rik is convincing me that we shouldn't expose mozOpaque or any other explicit subpixel AA control to the Web. It will be very easy for Web authors to test it in one place and discover that it works without realizing that they're causing problems for some users.
I think a fully automatic solution that tries to use subpixel AA but is always able to render grayscale AA if needed is the way to go. Possibly with an author hint to suggest opting into a more expensive rendering path. Rob -- Wrfhf pnyyrq gurz gbtrgure naq fnvq, “Lbh xabj gung gur ehyref bs gur Tragvyrf ybeq vg bire gurz, naq gurve uvtu bssvpvnyf rkrepvfr nhgubevgl bire gurz. Abg fb jvgu lbh. Vafgrnq, jubrire jnagf gb orpbzr terng nzbat lbh zhfg or lbhe freinag, naq jubrire jnagf gb or svefg zhfg or lbhe fynir — whfg nf gur Fba bs Zna qvq abg pbzr gb or freirq, ohg gb freir, naq gb tvir uvf yvsr nf n enafbz sbe znal.” [Znggurj 20:25-28]