One use case that we have which is not currently handled by XMLHttpRequest is incrementally sending data that takes a long time to generate _from the client to the server_. For example, if we were to record data from a microphone, we couldn't upload it in real time to the server with the current API.
The MediaStreaming spec also mentioned several use cases which would require streaming request data via an API: - Sending the locally-produced streams to remote peers and receiving streams from remote peers. - Sending arbitrary data to remote peers. http://www.whatwg.org/specs/web-apps/current-work/multipage/video-conferencing-and-peer-to-peer-communication.html - Wenbo