function centrar(){

    var elem = document.getElementById("foo"),

        elemWidth = elem.offsetWidth,

        elemHeight = elem.offsetHeight,

        winWidth = window.innerWidth,

        winHeight = window.innerHeight,

        winX = window.scrollX,

        winY = window.scrollY;

    elem.style.marginTop = (winHeight / 2 - elemHeight / 2) + winY + "px";

    elem.style.marginLeft = (winWidth / 2 - elemWidth / 2) + winX + "px";

}

​

document.addEventListener("DOMContentLoaded", centrar, false);

window.addEventListener("scroll", centrar, false);