Scroll down animation css (streak animation)

.css

.image-down-arrows span{
        position: relative;
        display: block;
        width: 3px;
        height: 8rem;
        background-color: #fff;
        animation: scrolldown 3.5s infinite;
    }

    @keyframes scrolldown {
        0% {
            height: 0;
            top: -8rem;
            transform: translateY(0);
        }
        50% {
            height: 8rem;
            top: 0;
            transform: translateY(0);
        }
        75% {
            height: 0;
            top: -8rem;;
            transform: translateY(8rem);
        }
        100% {
            height: 0;
            top: -8rem;
            transform: translateY(0);
        }
    }