.box_stretch {
  -webkit-animation-name: myfirst;
          animation-name: myfirst;
}



.box_reduce
{
    overflow: hidden;
animation: myfirst 3s;
-webkit-animation: myfirst 3s; /* Safari and Chrome */
}


@keyframes myfirst
{
    to {height: 100px;}
}

@-webkit-keyframes myfirst /* Safari and Chrome */
{
    to {height: 100px;}
}