I use this command I found on SO (was the first Google result): https://video.stackexchange.com/a/28276

    ffmpeg  -i input.mp4  -b:v 0  -crf 30  -pass 1  -an -f webm /dev/null
    ffmpeg  -i input.mp4  -b:v 0  -crf 30  -pass 2  output.webm

Making it two passes seems to be important for WEBM according to the post. It has worked well for me so far, but I need to look it up every time.