Positioning element at center of screen

Answer 1

With transforms being more ubiquitously supported these days, you can do this without knowing the width/height of the popup

.popup {
    position: fixed;
    top: 50%;
    left: 50%;
    -webkit-transform: translate(-50%, -50%);
    transform: translate(-50%, -50%);
}

Easy! JSFiddle here: http://jsfiddle.net/LgSZV/

Update: Check out https://css-tricks.com/centering-css-complete-guide/ for a fairly exhaustive guide on CSS centering. Adding it to this answer as it seems to get a lot of eyeballs.