Fluid css fold/unfold animation when having auto height

auto-height-folding.css

/*
 * Credits goes to +RichBradshaw, from whom I've learned this technique
 */
.container .hidden-container {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.2s ease; /* reverse transition*/
  /* remember not to put any borders, paddings here*/
}
.container:hover .hidden-container {
  max-height: 999px; /* enter bigger value if necessary, but don't over do as it impacts the animation speed! */
  transition: max-height 0.5s ease; /* forward transition */
}