Let's loot at:
$ avconv -qscale 1 -i x.mpg x.webm
The command above keeps the best quality available.
I turned a MPG file of about 300 MB into a WebM file of about 20 MB. I kept most of the video quality. The sound was good.
Please note that -qscale 1 should be the best quality.
Check the WebM file for the quality.
To me, WebM files are good for not-so-important videos, e.g. the video quality is not that important.
If the same images or similar images appear for more than five seconds, a WebM file seems to show the clear images, e.g. people's faces are shown clearly if their images last for five seconds or more in the video, or if these faces form a large part of the screen.