console 命令行工具 X clear
console
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"> 
<title>菜鸟教程(runoob.com)</title>
<style> 
  *{
      margin: 0;
      padding: 0
  }
  #main{
     display: flex;
     width: 400px;
     height: 400px;
     justify-content: center;
     align-items: center;
     border: 1px solid #ccc
  }
  #main div{
      width: 100px;
      height: 100px;
      background-color: red;
  }
  
</style>
</head>
<body>

<div id="main">
    <div></div>
</div>



</body>
</html>