But surely in that case it should be an
install dependency not a build dependency...
gah. videos in EPUBs.
...
I once got an ebook that turned out to be "enhanced" with a click-to-play audio button at the beginning. (Maybe because it was a short story they were trying to make it more "interesting".)
It played just fine in calibre's ebook-viewer, no qt5-multimedia at build or install...
But once I realized it was there, I edited it to delete it.
I never bother to check that kind of stuff

I regard it as an unhappy coincidence.