Center Horizontal and Vertically using Prototype

center-prototype.js

Element.addMethods({
  centerHorizontally:  function(element) {
    if (!(element = $(element))) return;

    var vpWidth = $(document).viewport.getWidth();
    var width = element.getWidth();

    element.style.left = (vpWidth / 2) - (width / 2) + 'px';
    return element;
  },  
  centerVertically: function(element) {
    if (!(element = $(element))) return;

    var vpHeight = $(document).viewport.getHeight();
    var height = element.getLayout().get('margin-box-height');
    var scrollTop = $(document).viewport.getScrollOffsets().top;

    var avTop = (vpHeight / 2) - (height / 2) + scrollTop;

    if (avTop <= 10) 
      avTop = 10; 

    element.style.top = avTop+ 'px';
    return element;
  }
});