数字越小，排列越靠前，默认是0.
注意：和 z-index 不一样。

例子

image.png

  1. <!DOCTYPE html>
  2. <html lang="en">
  4. <head>
  5.   <meta charset="UTF-8">
  6.   <meta http-equiv="X-UA-Compatible" content="IE=edge">
  7.   <meta name="viewport" content="width=device-width, initial-scale=1.0">
  8.   <title>Document</title>
  10.   <style>
  11.     div {
  12.       display: flex;
  13.       width: 500px;
  14.       height: 200px;
  15.       background-color: blue;
  16.       justify-content: center;
  17.     }
  19.     div span {
  20.       width: 100px;
  21.       height: 100px;
  22.       background-color: red;
  23.     }
  25.     /* 
  26.     div span:nth-child(3) {
  27.       align-self: flex-end;
  28.     } */
  30.     div span:nth-child(2) {
  31.       order: -1;
  32.     }
  33.   </style>
  34. </head>
  36. <body>
  37.   <div>
  38.     <span>1</span>
  39.     <span>2</span>
  40.     <span>3</span>
  41.   </div>
  42. </body>
  44. </html>