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The following command converts an mp4 video to webm:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 output.webm

When converting a video to Google’s VP9 video codec format with ffmpeg, one may receive the following error message:

Unknown encoder 'libvpx-vp9'

This indicates that FFMPEG was compiled without vpx-vp9 support. If ffmpeg was installed via homebrew, this can be fixed easily:

brew reinstall ffmpeg --with-libvpx

Afterwards video conversion will start properly.