<video> toggling video rendering

If you load <video> on some site and just want to hear the audio, is there  
a way to turn off the video output so you can save some cpu and just  
listen to the audio?

If not, I think that would be totally awesome if you could do that (wish  
youtube and all the other video sites did that).

Something like video.toggleVideoOutput() so the site can provide a button.  
Also, have the functionality present in the browser-provided controls.

I'm thinking you might be able to switch between <audio> and <video> with  
the same file (if <audio> can play the audio from the video file or use a  
separate audio file), but that wouldn't be as cool as just toggling the  
video output.

-- 
Michael