<div>W3TR</div>

div {
  background: green;
  width: 200px;
  height: 20px;
  transition: height;
  -webkit-transition: height; /* Safari 3.1 to 6.0 */
  -webkit-transition-duration: 3s; /* Safari */
   transition-duration: 3s;
}
div:hover {
  height: 100px;
}