Convert transparent webm video to transparent hevc video for safari on macos and ios. According to caniuse.com this gives 95.5% full support for browsers

script.sh

# Extract video frames, set fps to actual fps of the video
ffmpeg -c:v libvpx-vp9 -i video.webm -vf fps=30 frame-%03d.png

# Merge frames back into movie and encode it to hevc with transparency
ffmpeg -i 'frame-%03d.png' -c:v hevc_videotoolbox -allow_sw 1 -alpha_quality 1 -vtag hvc1 -vf fps=30 hevc.mp4

video.html

<video>
  <source type="video/webm" src="video.webm">
  <source type='video/mp4; codecs="hvc1"' src="hevc.mp4">
  <!-- Optional fallback without transparency -->
  <source type="video/mp4" src="fallback.mp4">
</video>