匹配属于父元素中唯一子元素的p元素 - 在线编辑器

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

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

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

  <p><b>注意:</b> Internet Explorer 8以及更早版本的浏览器不支持 :only-child选择器.</p>

</body>
</html>
        