PhotoPrism may use ffmpeg to transcode common video formats to MPEG-4 AVC. This container / codec combination doesn't require transcoding as it can be played natively by most modern browsers, see https://caniuse.com/mpeg4.
For a list of codecs ffmpeg supports, run this command in a terminal:
Note that Firefox may only play the corresponding AAC audio codec if the operating system supports it, see https://caniuse.com/aac.
All other browsers shouldn't have any issues.
While the QuickTime
.mov container format served as the basis for MPEG-4,
there are differences and the two are not quite interchangeable. They still
play in most browsers and won't be transcoded automatically for this reason.