Final Cut Pro
Alpha Channel, Transparency
Use prores 4444 codec to export or render files.
Convert to webm with alpha
ffmpeg -i video_in.mov -c:v libvpx -quality good -b:v 1M -qmin 10 -qmax 42 -maxrate 1M -cpu-used 0 -bufsize 2M -threads 4 -c:a vorbis -b:a 128k -strict -2 video_out.webm
Use cross desolve transition with chroma key filter.
Problem is that Keyed green screen appears during Cross Dissolve transition. You must use chroma key first to render files with alpha channel, only after that use transitions.
Using transition between clips inside compound clip don’t work if clips have chroma key filter enabled.