EricLaw’s IEInternals : COMET Streaming in Internet Explorer
One challenge with using XMLHTTPRequest is that browsers behave differently when it comes to streaming of content. A common complaint against XMLHTTPRequest in IE was that it doesn’t stream content. More specifically, the responseText property cannot be queried when readyState=3 (Receiving) and it only becomes available when readyState=4 (Loaded). This means that while your JavaScript will know when the transfer of the response body starts, you must wait for the response to complete before examining it. If your goal is to stream down a “never-ending” stream of data, you’ll find that IE’s XHR object never gets to the Loaded readyState and thus you cannot examine the “partial” response.