[nigeltao]

> I believe the standard does not specify what the intermediate progressive renderings should look like.

This is possibly getting too academic, but IIUC for a progressive JPEG, e.g. encoded by cjpeg to have 10 0xDA Start Of Scan markers, it's actually legitimate to post-process the file, truncating to fewer scans (but re-appending the 0xD9 End Of Image marker). The shorter file is still a valid JPEG, and so still relevant for discussing whether all decoders will render the same pixels.

I might be wrong about validity, though. It's been a while since I've studied the JPEG spec.