To do this you need to know the size of the element you are centering. Any measurement will do (i.e. px, em, percent), but it has to have a fixed size.
The css will look as follows:
// Replace X and Y with a number and u with a unit. do calculations
// and remove parens
.centered_div {
width: Xu;
height: Yu;
position: absolute;
top: 50%;
left: 50%;
margin-left: -(X/2)u;
margin-top: -(Y/2)u;
}
Edit: This centers in the viewport. You can only center in the browser window using JavaScript. But that might be good enough anyway, since you probably want to display a popup/modal box?