ffmpeg -i input.mp4 -c:v libvpx -minrate 1M -maxrate 1M -an -b:v 1M -ss 00:00:00 -t 00:00:10 -c:a libvorbis output.webm

-ss sets the start time, -t sets the duration, remember the -an to remove sound or you'll get an error message when you try to upload webm with sound to /b/