ffmpeg -i input.mov -c:v libvpx-vp9 -pix_fmt yuva420p -auto-alt-ref 0 -b:v 0 -crf 32 output.webm — or use the free ProRes to WebM converter.The FFmpeg command
FFmpeg reads the alpha of ProRes 4444 directly, so no special decoder is needed on the input side:
ffmpeg -i input.mov -c:v libvpx-vp9 -pix_fmt yuva420p -auto-alt-ref 0 -b:v 0 -crf 32 output.webm
Only the output needs care: -pix_fmt yuva420p keeps the alpha. Audio is not needed for overlays; add -an to drop it.
How much smaller?
ProRes is an intra-frame editing codec: in our 30-second 1080p test it was 374 MB. The WebM at -crf 32 was under 10 MB—dozens of times smaller—with edges intact.
Which MOV files work
ProRes 4444 and 4444 XQ, QuickTime Animation and other MOVs whose pixel format includes alpha (yuva…, rgba, argb). ProRes 422 has no alpha channel, so there is nothing to keep. Check with ffprobe -v error -show_entries stream=pix_fmt -of csv=p=0 input.mov.
The other direction
Going from WebM to ProRes needs one extra flag on the input: ffmpeg -c:v libvpx-vp9 -i input.webm -c:v prores_ks -profile:v 4444 -pix_fmt yuva444p10le -qscale:v 4 output.mov. Without -c:v libvpx-vp9 before -i, FFmpeg’s default VP9 decoder drops the alpha and the MOV comes out opaque. The -qscale:v 4 was about five times faster than the default in our tests with visually identical output.