You can modify Emacs so that it does something on every change in the current buffer, like for example save the change to disk if it's a file-backed buffer.

You could then have a script refreshing the browser on every file change (do it from outside Emacs, calling shell scripts from Emacs or anything 'non-Emacsy' from Emacs is too slow for the usecase here).

However you'll probably have to find a way for the 'save file on every buffer modification' to be very fast otherwise it's going to annoy you and prove impractical.

Btw I can't wait for someone to implement at least part of Emacs in JavaScript so that, eventually, users used to Emacs shall have the possibility to have a Web text editor that doesn't s*ck. Maybe using ClojureScript + the implementation of Emacs redone in Clojure (I think there's a project trying to do that right now).