A smooth transition with no invisible remnant or pointer-events: none hack.
It looks like in modern browsers a "position: fixed" will give elements an inherent z-index context based on their place in markup.
With this technique you can implement a "fullscreen" video which will always keep it's intended ratio.
A simple way to let image tags behave to a certain extent like background images with "background-size: cover".
Source: http://c2.com/cgi/wiki?FizzBuzzTest. Originally took me about 15 minutes.