<!--
 * @Author: ShuangxuNian
 * @Date: 2022-10-29 22:23:41
 * @LastEditors: ShuangxuNian
 * @LastEditTime: 2022-10-29 23:33:25
 * @Description: 
-->
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    html, body {
      margin: 0;
      border: 0;
      padding: 0;
      width: 100%;
      height: 100%;
    }
    .f {
      height: 100%;
      width: 100%;
      background-color: red;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
    }
    .a {
      height: calc(100% - 100px);
      width: 100%;
      background-color: yellow;
    }
    .b {
      height: 100px;
      width: 100%;
      background-color: green;
    }
  </style>
  <title>Document</title>
</head>
<body>
  <div class="f">
    <div class="a"></div>
    <div class="b"></div>
  </div>
</body>
</html>