Video textures and alpha transparency
WebM and HEVC/MOV formats are the only video formats that can be used on the web to display textures with alpha transparency.
WebM is supported in Chrome/Firefox/Edge/Samsung/Opera/Brave while HEVC/MOV is used on Safari (both desktop and mobile).
Selecting appropriate format
Use can use the following Puzzles snippet to select the video:
Converting WebM to HEVC
At the time of the writing, transparent videos are not supported in the stock libx265 encoder. Still you can use ffmpeg on Mac devices with hardware encoder called VideoToolbox to convert your videos:
ffmpeg -c:v libvpx-vp9 -i movie-webm.webm -c:v hevc_videotoolbox -alpha_quality 0.75 -vtag hvc1 movie-hevc.mov
You can install FFmpeg with Homebrew:
brew install ffmpeg