slide down linear css'
.hidden {
height: 0px;
-webkit-transition: height 0.5s linear;
-moz-transition: height 0.5s linear;
-ms-transition: height 0.5s linear;
-o-transition: height 0.5s linear;
transition: height 0.5s linear;
}
.hidden.open {
height: 200px;
-webkit-transition: height 0.5s linear;
-moz-transition: height 0.5s linear;
-ms-transition: height 0.5s linear;
-o-transition: height 0.5s linear;
transition: height 0.5s linear;
}