/* SET */

header {
    background-color: #98f;
    background-image: url("background-1.png"), url("background-2.png");
    background-size: auto 100%;
    background-repeat: repeat-x;
    background-position: center;
    animation: background 60s infinite;
}

/* ANIMATED */

@keyframes background {
    0%,100% {background-position-x: left, 0px;}
    50% {background-position-x: right, -500px;}
}