div {position: absolute;}

#container div {
    background-color: skyblue;
    border: 1px solid #333333;
    width: 500px;
    height: 100px;
    opacity: 0.5;
}

div#myBox {
    opacity: 1;
    background-color: gold;
    z-index: 1;
    -webkit-animation: mymove 10s infinite linear; /* Chrome, Safari, Opera */
    animation: mymove 10s infinite linear;
}




/* Chrome, Safari, Opera */
@-webkit-keyframes mymove {
    50% {z-index: 5;}
}

/* Standard syntax */
@keyframes mymove {
    50% {z-index: 5;}
}