html文件
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>登录</title>
<link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<script src="https://cdn.bootcss.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
<link rel="stylesheet" type="text/css" href="/static/register.css">
<body>
{#    <div id="tip1" class="alert alert-primary alert-dismissible fade" role="alert">#}
{#        <strong>成功!</strong> 绑定成功.#}
{#    </div>#}
{#    <div id="tip2" class="alert alert-warning alert-dismissible fade" role="alert">#}
{#        <strong>失败!</strong> 绑定失败.#}
{#    </div>#}

<h1 class="a">注册</h1>
<form action="/register" method="POST" class="middle">
    <p><input type="text" name="username" placeholder="输入用户名"></p>
    <p><input type="password" name="password" placeholder="输入密码"></p>
    <p><input type="password" name="repassword" placeholder="确认密码"></p>
    <p><input type="submit" value="注册"></p>
</form>

<form action="/" method="POST">
    <p class="b"><input type="submit" value="返回首页" color="grey"></p>
</form>

</body>
</html>
css文件
/* *{
  padding: 0;
  margin: 0;
  border: none;
} */
    /* margin: auto; =0 auto上下为0，左右自适应*/
/* body{
    display: flex;
    justify-content: center;
    align-items: center;
} */
/* body里middle内全部水平居中但注册、返回在middle类两边。middle则四个元素紧挨整体水平居中 */
/* .middle{
    display: flex;
    justify-content: center;
    align-items: center;
} */

/* Set document background and font */
body {
    background-color: #f8f9fa;
    font-family: 'Helvetica Neue', Arial, sans-serif;
  }
 
  /* Set form container styles */
  .middle {
    margin-top: 50px;
    padding: 30px;
    background-color: #fff;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    border-radius: 5px;
  }
 
  /* Set form header styles */
  .a {
    font-size: 36px;
    margin-bottom: 40px;
  }
 
  /* Set input styles */
  input[type="text"], input[type="password"] {
    margin-bottom: 25px;
    padding: 15px;
    border-radius: 3px;
    border: none;
    box-shadow: 0 2px 3px rgba(0, 0, 0, 0.1);
    width: 100%;
    box-sizing: border-box;
    font-size: 16px;
  }
 
  /* Set button styles */
  button[type="submit"] {
    background-color: #007bff;
    border: none;
    color: #fff;
    border-radius: 3px;
    padding: 15px 20px;
    font-size: 16px;
    cursor: pointer;
    transition: background-color 0.3s ease-in-out;
    width: 100%;
  }
 
  button[type="submit"]:hover {
    background-color: #0069d9;
  }
 
  /* Set responsive styles */
  @media only screen and (max-width: 767px) {
    .middle {
      margin-top: 20px;
    }
    .a {
      font-size: 24px;
      margin-bottom: 20px;
    }
    input[type="text"], input[type="password"] {
      padding: 10px;
      font-size: 14px;
    }
    button[type="submit"] {
      padding: 10px 15px;
      font-size: 14px;
    }
  }

/* Set document background and font */
body {
  background-color: #f8f9fa;
  font-family: Arial, sans-serif;
}

/* Set form header styles */
.a {
  margin-top: 50px;
  font-size: 36px;
  text-align: center;
}

/* Set form styles */
.middle {
  margin: 50px auto;
  max-width: 400px;
  padding: 30px;
  background-color: #fff;
  border-radius: 5px;
  box-shadow: 0px 2px 5px rgba(0,0,0,0.15);
}

/* Set input styles */
input[type="text"],
input[type="password"] {
  width: 100%;
  padding: 15px;
  margin-bottom: 20px;
  border-radius: 3px;
  border: 1px solid #ccc;
  font-size: 16px;
}

/* Set placeholder styles */
::placeholder {
  color: #bfbfbf;
}

/* Set submit button styles */
input[type="submit"] {
  width: 100%;
  background-color: #007bff;
  border: none;
  border-radius: 3px;
  padding: 15px;
  color: #fff;
  font-size: 16px;
  font-weight: bold;
  text-transform: uppercase;
  cursor: pointer;
  transition: background-color 0.3s ease-in-out;
}

input[type="submit"]:hover {
  background-color: #0056b3;
}

/* Set responsive styles */
@media only screen and (max-width: 767px) {
  /* Reduce font size for smaller screens */
  .a {
    font-size: 24px;
  }
  input[type="text"],
  input[type="password"] {
    padding: 10px;
    margin-bottom: 10px;
  }
  /* Change submit button padding */
  input[type="submit"] {
    padding: 10px;
  }
}
/* 第2种 */
/* Set document background and font */
body {
    background-color: #f8f9fa;
    font-family: Arial, sans-serif;
  }
 
  /* Set form header styles */
  .a {
    margin-top: 50px;
    font-size: 36px;
    text-align: center;
  }
 
  /* Set form styles */
  .middle {
    margin: 50px auto;
    max-width: 400px;
    padding: 30px;
    background-color: #fff;
    border-radius: 5px;
    box-shadow: 0px 2px 5px rgba(0,0,0,0.15);
  }
 
  /* Set input styles */
  input[type="text"],
  input[type="password"] {
    width: 100%;
    padding: 15px;
    margin-bottom: 20px;
    border-radius: 3px;
    border: 1px solid #ccc;
    font-size: 16px;
  }
 
  /* Set placeholder styles */
  ::placeholder {
    color: #bfbfbf;
  }
 
  /* Set submit button styles */
  input[type="submit"] {
    width: 100%;
    background-color: #007bff;
    border: none;
    border-radius: 3px;
    padding: 15px;
    color: #fff;
    font-size: 16px;
    font-weight: bold;
    text-transform: uppercase;
    cursor: pointer;
    transition: background-color 0.3s ease-in-out;
  }
 
  input[type="submit"]:hover {
    background-color: #0056b3;
  }
 
  /* Set responsive styles */
  @media only screen and (max-width: 767px) {
    /* Reduce font size for smaller screens */
    .a {
      font-size: 24px;
    }
    input[type="text"],
    input[type="password"] {
      padding: 10px;
      margin-bottom: 10px;
    }
    /* Change submit button padding */
    input[type="submit"] {
      padding: 10px;
    }
  }