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by AnotherGoodName 433 days ago
MP4 allows the header at the start or the end.

It’s usually written to the end since it’s its not a fixed size and it’s a pain for recording and processing tools to rewrite the whole file on completion just to move the header to the start. You should always re-encode to move the header to the start for web though.

It’s something you see too much of online once you know about it but mp4 can absolutely have the header at the start.