样式设置高度撑bai满整个页面，那么必须从最外层，一层一层设置高度100%；

<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    
    <style>
      * {
        padding: 0;
        margin: 0;
        box-sizing: border-box;
      }

      html,
      body,
      div {
        height: 100%;
      }

      .app {
        background: #ccc;
      }
    </style>
  </head>
  <body>
    <div class="app"></div>
  </body>
</html>