居中

1. 相对浏览器窗口，绝对上下左右居中

   .loading {
            width: 100%;
            height: 100%;
            position: fixed;
            z-index: 100;
            background: #fff;
        }
   .pic {
            width: 50%;
            height: 50%;
            margin: auto;
            position: absolute;
            top: 0; left: 0; bottom: 0; right: 0;
            background: red;
        }
 <div class="loading">
      <div class="pic">
      </div>
 </div>