<div class="floating"
     style="height: 50px; width: 50px; background: rgb(200,200,200);">
</div>

.floating {  
    animation-name: floating;
    animation-duration: 3s;
    animation-iteration-count: infinite;
    animation-timing-function: ease-in-out;
    margin-left: 30px;
    margin-top: 5px;
}

@keyframes floating {
    from { transform: translate(0,  0px); }
    65%  { transform: translate(0, 15px); }
    to   { transform: translate(0, -0px); }    
}