Don't do it.
No matter how tempting the urge, DON'T.
Dear Diarists,
Please, for the love of everything good in the world, do not embed YouTube videos with autoplay turned on. I just read a really well put together diary but auto-starting an accompanying video ruined his presentation.
A lot of us are at work or listen to music or whatever. If we want to watch a video we'll press play. If you decide to force us to watch it we may annoy our workmates with loud talking or music from your video. It's so freaking rude.
Also, if you set your video to autoplay and we post a comment in your video guess what? Two simultaneous streams play at once, at different intervals.
So please, don't autoplay youtube videos. Please.
PS: Daily Kos developers - could you write code to look for the autoplay option in the embed tags and strip it for us?