Runder is a rudimentary clone of CodePen that I wrote to see how much I could accomplish in a span of 24 hours.
All in all, it was a rewarding experience. I also got to present Runder at NUS Hackers last Friday!
I’d definitely look into the option of including libraries, such as Bootstrap and jQuery. This might be useful for people to get acquainted with libraries without having to do so locally.
Also, could it be possible to do client-side generation of encoded URLs so that users can share their code?