Very interesting and innovative new CSS background implementation proposed by Mozilla for use in Firefox 4. Basically you can use any element via it’s ID as easily as this:
It works with any HTML element even <iframe> or <video>. Make sure you download the Firefox 4 Beta and check out some of the demos, especially the animated water ripple reflection and the video demo. Both demos are probably a little over the top but certainly demonstrate the implementation effectively. Hopefully other browsers will pick this up and implement it too.