HTTP “200 OK” response

A complete request is not necessarily a successful request, and
your handler for the load event should check the status code
of the XMLHttpRequest object to ensure that you received an
HTTP “200 OK” response rather than a “404 Not Found” response,



for example.
There are three ways that an HTTP request can fail to complete,
and three corresponding events. If a request times out, the
timeout event is triggered. If a request is aborted, the abort
event is triggered. Finally, other network errors, such as too
many redirects, can prevent the completion of a request, and
the error event is triggered when this happens.