Python (and others) in Web Browser

Skulpt 

Skulpt is an entirely in-browser implementation of Python.
No preprocessing, plugins, or server-side support required, just write Python and reload.

repl.it - Select a Language

Various programming languages all running directly in Web Browser, 

thanks to compiling to optimized subset of JavaScript.

Such compiling is usually done by this award winning compiler

The LLVM Compiler Infrastructure Project

Software System Award - Award Winners: List By Year @ ACM

Interactive Textbook — How to Think like a Computer Scientist
(by using Python in a web browser)