以下是两个 <div> 元素添加 box-sizing: border-box; 属性的简单实例。 - 在线编辑器

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <style>
    .div1 {
      width: 300px;
      height: 100px;
      border: 1px solid blue;
      box-sizing: border-box;
    }

    .div2 {
      width: 300px;
      height: 100px;
      padding: 50px;
      border: 1px solid red;
      box-sizing: border-box;
    }
  </style>
</head>
<body>

  <div class="div1">两个 div 现在是一样大小的!</div>
  <br>
  <div class="div2">蜜蜂教程!</div>

</body>
</html>
​x
 
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <style>
    .div1 {
      width: 300px;
      height: 100px;
      border: 1px solid blue;
      box-sizing: border-box;
    }
​
    .div2 {
      width: 300px;
      height: 100px;
      padding: 50px;
      border: 1px solid red;
      box-sizing: border-box;
    }
  </style>
</head>
<body>
​
  <div class="div1">两个 div 现在是一样大小的!</div>
  <br>
  <div class="div2">蜜蜂教程!</div>
​
</body>
</html>