<!DOCTYPE html>
<html lang="zh-cmn-Hans">

<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">
  <title>Title</title>
</head>
<style>
  * {
    padding: 0;
    margin: 0;
  }

  #top {
    display: flex;
    background-color: aliceblue;
    height: 50px;
    align-items: start;
    justify-content: center;
  }

  #middle {
    display: flex;
    height: calc(100vh - 100px);
  }

  #left {
    display