While working on Swoooosh I stumbled across this draft from the W3C regarding “Progress Events 1.0”. The specification defines a set of standardized progress events and is intended for use with XMLHttpRequest and MAE. From their Abstract:
This document describes event types that can be used for monitoring the progress of an operation. It is primarily intended for contexts such as data transfer operations specified by XMLHTTPRequest [XHR], or Media Access Events [MAE].