.ball {
    text-align:center;
    margin: 15% 0% 0 50%;
    width: 100px;
    height: 100px;
    border-radius: 50%;
    background-color: #4267bd;
    /* Use cube-beizer.com  */
    animation: down 0.5s cubic-bezier(.5,0.05,1,.5);    ;
    animation-direction: alternate;
    animation-iteration-count: infinite;
}

@keyframes down {
    from { transform: translateY(0px); }
    to { transform: translateY(100px); }
}