[MayeulC]

> And the logistical costs of transcoding it to N different resolutions times M different codecs.

If only we had easy access to scalable video (SVC). If a container format supported it, the web browser could perform range queries to get the interesting bits as needed, no need for additional code.

And the uploader would need to transcode only once. This doesn't solve the codecs issue, but I think you could get away with offering one or two common codecs.