ffmpeg -i input.mkv -ss TIME_START -to TIME_END -vf scale=HOZ_RES:-1 -c:v libvpx -b:v 3*1024*1024*8/DURATION_SEC*0.975 -quality good -cpu-used 0 -bufsize 1000k -an -pass 1 -f webm tmp.fuck && ffmpeg -i input.mkv -ss TIME_START -toTIME_END -vf scale=HOZ_RES:-1 -c:v libvpx -b:v 3*1024*1024*8/DURATION_SEC*0.975 -quality good -cpu-used 0 -bufsize 1000k -an -pass 2 -f webm output.webm

replace the shit in caps with your own values