You use the onbeforeunload event, which doesn't block the event thread. (EDIT: actually it does, just not while JS code is executing, unlike alert().)

In fact, the HTML5 spec permits browsers to disregard the normal blocking behavior of alert()/confirm()/prompt() (making them no-ops) while this event is being handled: https://developer.mozilla.org/en-US/docs/DOM/window.onbefore... the dialog box is open

I managed to break that link somehow and can no longer edit it, here's the correct one: https://developer.mozilla.org/en-US/docs/DOM/window.onbefore...

Save it in drafts and notify them when they come back.

So Facebook and Twitter should also save drafts of posts you were going to make? What if latency is non-trivial, or connection is lost? Do I also want to send ajax posts for drafts every second to ensure that as little as possible is lost? A lot of this is mitigated by a very simple

  confirm("Are you sure you want to leave?");

Yeah but for all the times when I really do want to leave, now I have to annoyingly click "Yes" in a dialog I didn't expect in the first place.

I like the localStorage solution for drafts much better.

Twitter actually does save drafts in localStorage.