.myheader {
    /*height:80px;
    background:lime;*/
    position: fixed;
    top:0;
    right:0;
    left:0;
    transition: transform .25s .1s ease-in-out;
    z-index: 1;
 }
 
 .down .myheader {
    transform: translate3d(0, -80px, 0); /* um 80px nach oben verschieben */
 }