I started working on something like that, a text editor based on Ace (the text editing component from Cloud 9) but with Dropbox sync (actual sync, so it works offline) a few weeks ago (mostly for my Chromebook, and Cloud 9 sort of requires you to install Node which poses some difficulties on a chromebook). https://github.com/antimatter15/cloudfall
Is this ide very popular? I have never heard from anyone who's really used it enough. I have some experience with just the Ace editor, and found it to be great. But c9 itself felt slow and uncomfortable.
It's been trough a huge rewrite, and tere are a large number of plugins now. The hosted version of it comes with a VM, so you can run any server side thing you normally would.
If you haven't tried it lately, how it a go. If you have, and it is still slow, open up a bug ticket and help make it better.
Light Table won't ultimately be in the "browser" - it will be in a wrapped webview. The experience on mac right now is that you have a standalone app just like you would with sublime et. al. Once we get some time, the same will be true for linux and windows :)
Along a similar line, I'd love it if I could run an IPython Notebook instance in a packaged app for either Firefox or Chrome. There's a lot of little annoyances to using the notebook interface in my main browser, like having my notebook tabs mixed in with my general browsing tabs, not having a separate entry in my Alt-Tab list, etc. Using something like Prism and its modern descendants would help make IPython that much better, IMHO.