This page has a counterpart, Theora, for Ogg Theora video.
MEncoder
mencoder -ovc lavc -oac lavc ./19--Offline-OpenStreetMaps-in-the-context-of-Electronic-Election-Observation.wmv -o output.webm -of lavf -lavfopts format=webm -lavcopts acodec=vorbis:vcodec=libvpx -ffourcc VP80
Confirmed working with r31918. More information available with this mailing list thread.
