This indicates when the readyState property changes.
As well as the JScript syntax, there are two alternatives:
Inline HTML:
<ELEMENT onreadystatechange="myEventHandler" ... >
Named Script (IE Only):
<SCRIPT FOR = object EVENT = onreadystatechange>
The onreadystatechange event indicates when the readyState property changes.
The handler for this event can be explicitly set with the onreadystatechange property.