I can see this being useful for introductory programming classes, but not for real projects. For a real programming project, there are a lot of things you need

I can see this being useful for introductory programming classes, but not for real projects. For a real programming project, there are a lot of things you need which are very hard to implement in a browser: a debugger, profiler, version control, and code completion, to start. And then there's the whole issue of the host running people's arbitrary in-development code on their server, which may accidentally spin into an infinite loop, or deliberately send spam, exploit local root exploits on the host system and so on. The browser is good for many things, but it is not and will not ever be a good IDE.