Polyfill for compareDocumentPosition if you need that for old IE or something. But you could probably just use .contains() too.

compareDocumentPosition.js

  function compareDocumentPosition(thisNode, other) {
    function recursivelyWalk(nodes, cb) {
        for (var i = 0, len = nodes.length; i < len; i++) {
            var node = nodes[i];
            var ret = cb(node);
            if (ret) {
                return ret;
            }
            if (node.childNodes && node.childNodes.length) {
                var ret = recursivelyWalk(node.childNodes, cb);
                if (ret) {
                    return ret;
                }
            }
        }
    }

    function testNodeForComparePosition(node, other) {
        if (node === other) {
            return true;
        }
    }

    function simpleCompare(node, other) {
      // Check to see if any of other's parents
      // are the node we're checking.
      if(node.nodeType == Node.ELEMENT_NODE) {
        while (other = other.parentNode) {
          if(other === node) {
            return Node.DOCUMENT_POSITION_CONTAINED_BY +
                Node.DOCUMENT_POSITION_FOLLOWING;
          }
        }
      }
      // And the reverse.
      else if(other.nodeType == Node.ELEMENT_NODE) {
        while (node = node.parentNode) {
          if(other === node) {
            return Node.DOCUMENT_POSITION_CONTAINS +
                Node.DOCUMENT_POSITION_PRECEDING
          }
        }
      }
    }

    function identifyWhichIsFirst(node) {
        if (node === other) {
            return "other";
        } else if (node === reference) {
            return "reference";
        }
    }

    var reference = thisNode,
        referenceTop = thisNode,
        otherTop = other;

    if (this === other) {
        return 0;
    }
    while (referenceTop.parentNode) {
        referenceTop = referenceTop.parentNode;
    }
    while (otherTop.parentNode) {
        otherTop = otherTop.parentNode;
    }

    if (referenceTop !== otherTop) {
        return Node.DOCUMENT_POSITION_DISCONNECTED;
    }

    result = simpleCompare(thisNode, other);
    if(result) {
      return result;
    }

    var ret = recursivelyWalk(
        [referenceTop],
        identifyWhichIsFirst
    );
    if (ret === "other") {
        return Node.DOCUMENT_POSITION_PRECEDING;
    } else {
        return Node.DOCUMENT_POSITION_FOLLOWING;
    }
  }

No, we can't use .contains, because I've to create a .contains polyfill and my code looks like this:

// This polyfill only works in IE9+
if(!Element.prototype.contains){
    Element.prototype.contains = function(node){
        return !!(this.compareDocumentPosition(node) & 16);
    };
}