Run a player or a program as desktop background. More information: https://github.com/ujjwal96/xwinwrap.
xwinwrap -b -nf -ov -- mpv -wid wid --loop --no-audio --no-resume-playback --panscan=1.0 path/to/video.mp4
xwinwrap -b -nf -fs -ov -- mpv -wid wid --loop --no-audio --no-resume-playback --panscan=1.0 path/to/video.mp4
xwinwrap -b -nf -ov -o 0.8 --- mpv -wid wid --loop --no-audio --no-resume-playback --panscan=1.0 path/to/video.mp4
xwinwrap -g 1600x900+1920 -b -nf -ov -- mpv -wid wid --loop --no-audio --no-resume-playback --panscan=1.0 path/to/video.mkv