[BenoitEssiambre]

Right, however javascript will still be the only full featured language having its runtime and libraries included with the browser. That means that for other languages, only the ones with fairly light runtimes and libraries could be suitable for inclusion in most web apps. This will limit the choices for non javascript general development. It may be possible to do a webassembly/webgl rendering engine to replace html/css using a limited set of dependencies.

[akiselev]

Most news sources already load between 2 and 10 megabytes of Javascript, which is a significant fraction of, say, Python's runtime and many, many people browse those pages on mobile devices and connections without a problem. In Python's case, a large amount of the runtime size is taken up by all of the platform specific implementations of the standard library and they wouldn't work in the sandbox anyway.

Is there anything preventing Python.org from hosting the runtime environment on their CDN that everyone caches locally? Are there security issues from cross domain modules? Worst case, I don't think it would be hard to extend the LocalStorage API to cache modules between domains (perhaps with signed runtimes and a fallback).