[Jach]

When many websites are already shipping down megs of crap, I don't think the few extra megs of a python runtime would matter that much to the end user, especially if it's cached. Especially if it's marked by the browser as cross-site 'cached' (perhaps by verifying a signature and installing it as a plugin) so it's only done once per version. You can go further with more browser support, for instance browsers could bundle popular runtimes with their own distribution (Chrome + Flash).

[crooked-v]

> Especially if it's marked by the browser as cross-site 'cached'

That runs into the problems that have generally prevented browsers from cross-site caching common scripts already.