code: function() {
        while (this.dirty.length > 0) {
          var found = this.dirty.pop();
          // IE8 sometimes throws an unexplainable 'invalid argument'
          // exception for found.parentNode
          try {
            // If the node has been coloured in the meantime, or is no
            // longer in the document, it should not be returned.
            while (found && found.parentNode != this.container)
              found = found.parentNode;
            if (found && (found.dirty || found.nodeType == 3))
              return found;
          } catch (e) {}
        }
        return null;
      }
docs: Fetch one dirty node, and remove it from the dirty set.