So far, my impression was that the problem with building a Flash implementation isn't so much the language runtime as everything else - rendering, video, audio, input, etc. Isn't Tamarin [1] enough to run ActionScript 3?

FWIW, I realise that this has an advanced implementation of some graphical features and I'm not trying to belittle it, it seems an impressive effort. I'm just curious what's currently holding back a full implementation.

[1] http://en.wikipedia.org/wiki/Tamarin_(JavaScript_engine)