The First Public Working Draft of a specification focusing on Progress Events has been published by the Web API working group of W3C. Whenever data on demand is downloaded progress events are used for construction of loading bars, performing operations when data has been transferred completely and monitoring the progress. All the aspects of XMLHttpRequest object are not covered by the specification but the Progess Events 1.0 specification tends to focus on the five events and the matching interfaces which are defined for AJAX usage.

While making use of progress event there should be one start event which should be followed by zero or more progress event and then followed by an event which can be of type abort, error or load depending upon the intended outcome of the operation. The area which is focused is event types which can be used for monitoring the progress of operation and report feedback. You can check the working draft over here.