#container { 
      max-height: 500px;
      height: 90%;
      top: 5%;
      bottom: 5%;
      margin-top: auto;
      margin-bottom: auto;
}

* html #container { 
      max-height: none;
      height: 500px;
      top: 50%;
      margin-top: -260px;
      bottom: auto;
}

*+html #container { 
      max-height: none;
      height: 500px;
      top: 50%;
      margin-top: -260px;
      bottom: auto;
}