docs: function wheelDelta(event){
      var delta = 0;
  
      if ('deltaY' in event)
        delta = -event.deltaY;      // safari & gecko
      else
        if ('wheelDelta' in event)
          delta = event.wheelDelta; // IE, webkit, opera
        else
          if (event.type == 'DOMMouseScroll')
            delta = -event.detail;  // old gecko
  
      return delta && (delta / Math.abs(delta));
    }