selector{  
    animation-name: floating;
    animation-duration: 3s;
    animation-iteration-count: infinite;
    animation-timing-function: ease-in-out;
}

@keyframes floating {
    from { transform: translate(0,  0px); }
    50%  { transform: translate(0, 15px); }
    to   { transform: translate(0, -0px); }    
}