Convert mov with Alpha to VP9 Webm with Alpha Using ffmpeg

All of the other solutions resulted in a video of subpar quality. Please ensure that you define the bitrate to your liking. I changed mine from 1M to 2M and was satisfied.

ffmpeg -i "Model 1 V1.mov" -f webm -c:v libvpx -b:v 2M -acodec libvorbis -auto-alt-ref 0 model1v3.webm -hide_banner

If you're using After Effects or Premiere Pro, there's also this plugin, which can be used in Adobe Media Encoder.

Since 2016-07-13, it's possible to encode VP9/webm videos with alpha channel (VP9a). However, the command you use here will create a VP8a video. Assuming you got a copy of ffmpeg compiled after that date, all you need is change the libvpx to libvpx-vp9. You don't need the yuva420p conversion either (is selected by default).

Try

ffmpeg -i input.mov -c:v libvpx -pix_fmt yuva420p out.webm