alt Hacker NewsBecause to replicate the power of Emacs, you'd need to throw away a lot of privacy/security in the browser. You'd need to let one tab read the contents of another by default. A change to make your mail reading better may break your banking app (yes, that's a reality with Emacs).
Also, Emacs windows vs tabs. Think of having each tab be a non-overlapping window within the browser window.
Finally, most browsers don't let JS code modify the browser itself. If I suddenly decide I'd like Firefox's menu completely rearranged, I don't think JS will give it to me.
If I have two editor tabs open as two browser tabs they should already be able to talk if they're from the same origin, which different editor tabs would be.
The JS code won't be able to modify the browser itself, but neither can ELisp modify the actual Emacs binary code. The customizability in both cases is really coming from an "OS" being implemented in user-controlled scriptspace. You'd have just as much ability to customize that HTML-and-JS OS if it was running in the browser as you have to customize Emacs I think...