数字时钟(单文件版本)，HTML代码在线实时编辑、实时预览

            
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            <!DOCTYPE html>
<html>
   <head>
       <title>待会写标题</title>
       <meta charset="UTF-8">
       <meta name="viewport" content="width=device-width, initial-scale=1.0">

       <style>
           body {
              margin: 0;
           }
       </style>

<style>
  *, *::before *after {
    font-family: inherit;
    box-sizing: inherit;
    margin: 0;
    padding: 0;
  }

  html {
    font-size: 0.75vmin;
    margin: 0;
    height: 100vh;
    overflow: hidden;
    display: flex;
    flex-direction: column;
    align-content: center;
  }