I was wondering how all this could work in a browser, and to my surprise it appears there's a pretty well-developed project that has a bunch of language interpreters working in the browser via Emscripten (discovered via viewing coderpad.io's source): http://replit.github.io/jsrepl/
It appears to have been developed by/for http://repl.it/, which is education- rather than interview-oriented.
Repl.it is really quite great. Unfortunately it is beginning to feel its age (2 years, now). I'm looking to get off of it for some of the core languages I support, and I already rely on other services for the other, trickier languages (C++, java et al).
It appears to have been developed by/for http://repl.it/, which is education- rather than interview-oriented.