I don't wanna down a nice looking project, very well done on the interface but, another one? There are already like 7+ of these I don't really see the need to produce another one especially one that is HTML+CSS only when many of the others feature JavaScript and more.
I think the key extra feature is that you can share what you're working on with someone else and have both people collaborate in real-time (i.e. changes on either end show up near-instantly on the other side). Supplemented by a voice channel, this could potentially be useful.
JSFiddle et. al. don't have this, but I'm sure there are others out there that do.
Because none of them are the right solution for every use case. http://cssdeck.com/ comes closest but OP's solution is much more simple and to-the-point.
What features do you provide that say, http://jsfiddle.net/, http://cssdesk.com/, http://dabblet.com/, http://rendera.heroku.com/, doesn't? (Just the first 4 items that came up in a Google search, the list could go on...)