#reading-progress-bar {
    position: fixed;
    top: 0;
    left: 0;
    height: 4px;
    width: 0%;
    z-index: 9999;
    transition: width 0.2s ease-out;
    background: linear-gradient(to right, red, orange, yellow, green, blue, indigo, violet);
    border-radius: 2px;
    border: none;
}