Turn off display for YouTube?

[Willyammer]

There are a bunch of lectures on YouTube where I don't have to (and don't want to) watch the video. How do I turn off the display and save battery time?

 

[PaulL]

I know that tapping the on/off button to put the screen to sleep also stops the video ...
How about downloading the YouTube vid with KeepVid.com to an mp4 fprmat, then playing with the media player, where I think the sleep button doesn't shut off the playback?