Dig this - network events into the browser... who'd a thunk it? (Other than the half dozen companies doing it over the past five years.)
Secondly, the browser isn't a good listener to external events. If you want to build an application, for example, to show you instantly when someone bids or a price changes, it is hard. You can poll, but poll too frequently and the application starts to feel sluggish and it isn't easy to do this. What you really want is an event driven model where in addition to the events like typing the page can describe events like an XMPP message or a VOIP request or a data-changed post for an ATOM feed.