GStreamer's `gst-launch` can convert video to `webm` from the command line.  
The following converted a test `mp4` to `webm` (vp8). I've never used it before, and I haven't got the audio options right (no audio), but the video works fine.   

This info is basically from [a Stack Overflow post](http://stackoverflow.com/questions/4649925/convert-video-to-webm-using-gstreamer).. The `mux` arg in that example caused a syntax error, so I tried `oggmux` as shown in a `man gst-launch` example... It ran, but without audio, so it needs some audio option tweaking.   


    gst-launch \
        filesrc location="$if" ! \
        decodebin name=demux ! \
        queue ! \
            ffmpegcolorspace ! \
            vp8enc ! \
            webmmux name=mux ! \
            filesink location="$if".webm demux. ! \
        queue ! \
            progressreport ! \
            audioconvert ! \
            audioresample ! \
            vorbisenc ! \
            oggmux