Have you find it’s a difficult to reduce the z-index for your youtube video here is simple solution to make your video behind the website content.
Usually youtube videos has a more z-index than the other contents. Lowering the z-index is not a good trick instead of that you can use wmode parameter to embed your video
You can refer the following code
<embed width='560' height='315' src='http://www.youtube.com/v/5LHpwwU70FI&hl=en&fs=1' type='application/x-shockwave-flash' allowfullscreen='true' allowscriptaccess='always' wmode="opaque">