/* animation monte et descend */
main .up-down {
    width: 100%;
    height: auto;
    display: block;

    animation: floatUpDown 3s ease-in-out infinite;
}

@keyframes floatUpDown {
    0% {
        transform: translateY(0);
    }

    50% {
        transform: translateY(-20px);
    }

    /* monte de 20px */
    100% {
        transform: translateY(0);
    }
}