Last week, Opera posted a blog entry describing a different style of doing server push. The technique described is based on server side DOM events as currently proposed in the WHATWG Web Applications 1.0 specification. Also check out the demo, which is based on an early and partial implementation of server side events in Opera 9.x. The approach described can be used to send events over long-running connections to the browser, and in someways resembles the onload event handler of the XMLHttpRequest as implemented in Firefox. But there are some key differences between these approaches.

Ever wonder why the method getElementById returns null when applied to responseXML of XMLHttpRequest?

Thomas Rabaix discusses about this problem briefly in his Issues when developing AJAX Libraries. Here is a more detailed answer.