<div class='line'></div>

.line{
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translateX(-50%) translateY(-50%);
    width: 80%;
    height: 2rem;
    border-radius: 1rem;
    overflow: hidden;
    background: linear-gradient(to left, green, white);
}

.line::after {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
    background: linear-gradient(to right, green, white);
    animation: my-animation 3s linear infinite alternate;
}

@keyframes my-animation {
    from {
        opacity: 0;
    }
    to {
        opacity: 1;
    }
}