3box is an experimental Web-based programming environment, comparable to a text editor with some REPL-like features.
It is built of continuously-updated UI elements called feedbacks.
Feedbacks can display the output of a function, and other useful information, updated as the code is written.
3box is in very early days.
An earlier post describes somewhat related goals.
There is a small demo which also serves as a tutorial and introduction to the features implemented so far.
To start create your own 3box instance and you'll be taken your copy of the tutorial, which you can then bookmark and play with and modify as much as you like.
3box has a mailing list for general discussion and new version announcements.