:only-child

如果某个元素是其父元素的唯一子元素，那么它就会被选中。

如果某个父元素还有其它的子元素，那么它将不会被匹配到。

示例:

Change the text and add a border for each button that is the only child of its parent. 若每个 div 中只有一个按钮，则改变这些按钮的外观。

<!DOCTYPE html>
<html>
<head>
<style>
  div { width:100px; height:80px; margin:5px; float:left; background:#b9e }
</style>
<script src="jquery.min.js"></script>
</head>
<body>

<div>
  <button>Sibling!</button>
  <button>Sibling!</button>
</div>

<div>
  <button>Sibling!</button>
</div>
<div>
  None
</div>

<div>
  <button>Sibling!</button>
  <button>Sibling!</button>
  <button>Sibling!</button>

</div>
<div>
  <button>Sibling!</button>
</div>

<script>


  $("div button:only-child").text("Alone").css("border", "2px blue solid");


</script>
</body>
</html>

演示: