匹配属于同类型中唯一同级元素的p元素的背景颜色 - 在线编辑器

          <!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <style>
    p:only-of-type {
      background: #ff0000;
    }
  </style>
</head>
<body>

  <div>
    <p>This is a paragraph.</p>
  </div>

  <div>
    <p>This is a paragraph.</p>
    <p>This is a paragraph.</p>
  </div>

</body>
</html>
        