/* SET */

header {
    background-image: url("background-1.png"), url("background-2.png"), url("background-3.png");
    background-size: unset;
    background-repeat: repeat-x;
    background-position: center bottom;
    animation: background linear 120s infinite;
}

/* ANIMATED */

@keyframes background {
    0% {background-position-x: 0px, 0px, 0px;}
    100% {background-position-x: -15360px,-10240px,-5120px;}
}