There is some algorithm of showing "loading" indicator in browsers. There are even some techniques to trigger this indicator, such as:
Display *browser* loading indicator (like when a postback occurs) on ajax calls
Is there any documentation of it from any of the browser teams?
I assume it's user-agent business and there is nothing in w3c specs about it.
Is it connected with document readiness (value of readystatechange) and how? What about xmlhttprequests? Initial resource loading?