> Wouldn't you just hit undo?

Because:

1. Undo is usually treated as an application-level concern, meaning that once the application has exited there is no specific undo, as it is normally though of, function available. The 'desktop environment' integration necessary for this isn't commonly found.

2. Even if the application is still running, it only helps if the browser has implemented it. You mention Chrome has it, which is good, but Chrome is pretty lousy about just about everything else, so... Pick your poison, I guess.

3. This was already mentioned as the better user experience anyway, albeit left open-ended for designers, so it is not exactly clear what you are trying to add. Did you randomly stop reading in the middle?