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;
}