gistfile1.js

Element.Methods.descendantOf = (function() {
  var doc = document.documentElement;
 
  // DOM Level 3 approach (Gecko, Opera)
  if ('compareDocumentPosition' in doc) {
    return function(element, ancestor) {
      return (element.compareDocumentPosition(ancestor) & 8) === 8
    };
  }
  // IE, Safari
  if ('contains' in doc) {
    return function(element, ancestor) {
      return ancestor !== element && ancestor.contains(element)
    };
  }
  // fallback on while loop
  return function(element, ancestor) {
    while (element = element.parentNode)
      if (element === ancestor) return true;
    return false;
  };
})();