.improtant {
         color:red;
}
body  {
    background-image: url('https://images.cnblogs.com/cnblogs_com/afeili/1765790/t_200515075257moon.jpg?a=1589533408906');
   <--!background-size:100% 100%;-->
}
 .clocks {
            height: 500px;
            margin: 25px auto;
            position: relative;
            width: 500px;
        }
        img {
            width: 0;
            height: 0;
        }