html::before {
    content: '';
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;            /*  此时 -1 可以正常工作 */
    background-image: url(https://images.cnblogs.com/cnblogs_com/blogs/878359/galleries/2531551/o_260912092424_Acid.jpg);
    background-size: cover;
    background-position: bottom;
    /* 关键：用 box-shadow 模拟背景变淡，而不是 opacity */
    box-shadow: inset 0 0 0 2000px rgba(0, 0, 0, 0.93); /* 0.3 是暗度，可调 */
    pointer-events: none;
}