ffmpeg conversion - vp8 with alpha
I took the sample from apple https://developer.apple.com/videos/play/wwdc2019/506/
My command line using on colab was
!ffmpeg -y -i https://devstreaming-cdn.apple.com/videos/wwdc/2019/506lqy7sprpfyo800/506/506_hd_hevc_video_with_alpha.mp4 -s 960x540 -vcodec vp8 -metadata:s:v:0 alpha_mode=&quot;1&quot; c0016_alpha.webm

Hope it matches your scenario.