It's not always possible in Flash, as SWFs published for newer players are sometimes completely incompatible with older versions of the player - usually it's done with JS on the page it's embedded on.

Besides, this is just an experiment - I bet you don't see those canvas demos taking these things into account!

This is one more reason that flash is bad. Adobe makes truly terrible software. Sure, it makes pretty images and stuff, but it's horribly written and architected.