Here’s a short snippet to set the opacity of an element:

function setOpacity(e,opacity){
  var o=e.style;
  o.opacity=(opacity/100); //Opera
  o.MozOpacity=(opacity/100); //Mozilla+Firefox
  o.KhtmlOpacity=(opacity/100); //Konqueror
  o.filter="alpha(opacity="+opacity+")"; //IE
}

var e=document.getElementById('myElement');
setOpacity(e, 50);//Sets the opacity to 50%