MJPEG in casual parlance is more of a "technique" than a format -- and multipart/x-mixed-replace is a very loose definition of even something as loosely defined as MJPEG.
Out of video containers, QuickTime introduced "Motion-JPEG" [1] with each frame in the container being a JPEG, and there have been formal definitions of how to communicate "JPEG-compressed video" with network protocols, like in RTP [2].
Meanwhile mixed-replace was a Netscape trick [3] to do server push with HTTP that never became a real force but was reimplemented by almost every browser trying to stay Mozilla-compatible (except IE), and happily used by webcams for dead-simple videostreaming.
Out of video containers, QuickTime introduced "Motion-JPEG" [1] with each frame in the container being a JPEG, and there have been formal definitions of how to communicate "JPEG-compressed video" with network protocols, like in RTP [2].
Meanwhile mixed-replace was a Netscape trick [3] to do server push with HTTP that never became a real force but was reimplemented by almost every browser trying to stay Mozilla-compatible (except IE), and happily used by webcams for dead-simple videostreaming.
[1] https://developer.apple.com/standards/qtff-2001.pdf [2] https://tools.ietf.org/html/rfc2435 [3] https://web.archive.org/web/19981203153836/fishcam.netscape....