Earlier quoted context omitted.
It's just a start, I'm sure it wouldn't be too difficult to add these features in future for popular languages if there is demand for it. It's pretty exciting IMHO.
Executing code would actually be very difficult. They would need to configure an isolated VM for each run (for security) and somehow figure out a generic way to install/configure dependencies.
It's fairly common actually, like the PHPs running in shared environements.
Many programming language's websites let you try running some code from the browser, without having to install anything as well, see for instance http://dlang.org/ (amongst many others).
I'm not sure it would be very useful (to me at least, but I mostly write C) but it's definitely not rocket science.