.scan-line {
    position: absolute;
    left: 0;
    width: 100%;
    height: 2px;
    background: red;
    animation: scan 1.2s infinite linear;
    z-index: 2;
}

@keyframes scan {
    0% {
        top: 30%;
    }

    50% {
        top: 50%;
    }

    100% {
        top: 70%;
    }
}