Or, you know, run emacs on the server and write a "ssh based client" for it. :)
What would be the value of running in a browser specifically? The files you are editting almost by definition live outside the browser's sandbox. And if they're going to be managed server-side, it seems like there's a perfectly working 40 year old paradigm for managing files on a remote host.
The other major challenge is just duplicating javascript the sheer quantity of functionality emacs provides. An easier first step may be to run emacs on the server and write a javascript-based client for it.