I don't know but think that what you are after is something to edit the code of a web app. If it is then since the web app is essentially a web page you could use any text editor, I personally like Gusto for the iPad- http://itunes.apple.com/us/app/gusto/id364906873?mt=8.
Either that or you are looking for a HTML editor which is a web app itself. In that case, sorry I don't know of any.
