The response of the request is available in two formats: as raw text and as an XML document.
The responseText property is empty if the state is 0 through 2—that is, no data has been
received yet. When the state transitions to 3 (receiving data), the property contains the data
received so far, interpreted using the character encoding specified in the response. If no character encoding was specified, it employs UTF-8.
successfully parsed to an XML document. If the body of the response is not XML or if the
parsing fails for any reason, the property returns null. It is important to note that the construction
of the XML document takes place on the client once the raw HTTP response has been
fully received.
