css之页面顶部阴影

body:before {
          content: "";
          position: fixed;
          top: -10px;
          left: 0;
           100%;
          height: 10px;
 
          -webkit-box-shadow: 0px 0px 10px rgba(0,0,0,.8);
          -moz-box-shadow: 0px 0px 10px rgba(0,0,0,.8);
          box-shadow: 0px 0px 10px rgba(0,0,0,.8);
 
          z-index: 100;
}