/* 背景 */
.bg::before{
   content: '';
   position: fixed;
   background-image: url('../img/bk.png');
   top: 0;
   left: 0;
   width: 100%;
   height: 100vh;
   height: 100svh;
   background-size: cover;
  z-index: -1;
}
.bg-demo{
  width: 100%;
  height: 100%;
}