function targetBlank(event) {
  event = event ? event : window.event;
  var target = event.srcElement ? event.srcElement : event.target;

  while (target.nodeName.toLowerCase() != "a" && target.parentNode != null)
    target = target.parentNode;

  window.open(target.getAttribute("href"), "_blank");

  // DOM
  if (event.cancelable)
    event.preventDefault();

  // IE
  return false;
}

function targetBlankBind() {
  var tags = document.getElementsByTagName("A");
  var i;
  for (i=0; i<tags.length; i++)
    if (tags[i].getAttribute("rel") == "external")
      tags[i].onclick = targetBlank;
}

if (window.attachEvent) {
  // IE
  window.attachEvent("onload", targetBlankBind);
}
else {
  // DOM
  window.addEventListener("load", targetBlankBind, false);
}