Lot I don't know about how browsers are shipped, but it seems to me like browsers could easily get away with packing in a few languages and their STLs as part of their default installs. Python is what, 25MB? Would another couple hundred megs of disk space be such a big deal?
Possibly – if you can find a single version of Python that everybody will be happy with, forever.
Being able to cache runtimes and libraries like that across sites would be nice, though (but probably enables fingerprinting, so one Python runtime per origin it is).
