how to encode webm video files for html5 streaming

ffmpeg -i video.mp4 -s 1280x720 -vpre libvpx-720p -b 3900k \
        -pass 1 -an -f webm -y video.webm

ffmpeg -i video.mp4 -s 1280x720 -vpre libvpx-720p -b 3900k \
        -pass 2 -acodec libvorbis -ab 100k -f webm -y video.webm