Generally used to center a dialog (a div element).
Usage:
Position.Center(dialog);
The element can also be centered relatively to another node by using the parent parameter.
It requires the this snippet

Position.Center = function(element, parent) {
        var w, h, pw, ph;
        var d = Element.getDimensions(element);
        w = d.width;
        h = d.height;
        Position.prepare();
        if (!parent) {
                var ws = Position.GetWindowSize();
                pw = ws[0];
                ph = ws[1];
        } else {
                pw = parent.offsetWidth;
                ph = parent.offsetHeight;
        }
        element.style.top = (ph/2) - (h/2) -  Position.deltaY + "px";
        element.style.left = (pw/2) - (w/2) -  Position.deltaX + "px";
}