Here, I'll make a tutorial.
Use my profile as an example.
Here's the HTML I added.
<object width="0" height="0"><param name="movie" value="//www.youtube-nocookie.com/v/POwaPdAPyNs?version=3&hl=en_US&rel=0;autoplay=1" /><param name="allowFullScreen" value="true" /><param name="allowscriptaccess" value="always" /><embed src="//www.youtube-nocookie.com/v/POwaPdAPyNs?version=3&hl=en_US&rel=0;autoplay=1" type="application/x-shockwave-flash" width="0" height="0" /></object>
I used the 'Embed' option on youtube, and used the old embed code because for some reason the new just didn't work. Anyway, there are 4 things you need to change, 2 are just doubles.
- There are 2 widths and 2 heights. Make them both "0"
- There is also 2 links, both to the video you chose. At the end of their links, add this exactly ";autoplay=1" it will add a parameter to the link for autoplaying the video, and 1 means it's true
Note: A few videos may not allow embeding, so if your video doesn't work then try a different video.