:only-child 选择器
only-child selector
示例
针对每个属于它的父元素的唯一子元素的按钮改变文本，添加边框。
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>only-child demo</title>
<style>
div {
width: 100px;
height: 80px;
margin: 5px;
float: left;
background: #b9e;
}
</style>
<script src="https://code.jquery.com/jquery-1.10.2.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>
演示结果