唯一子伪类选择器示例
:only-child CSS 伪类表示任何元素，它是其父元素的唯一子元素。
HTML：
<div>
<p>This paragraph is the only child of the div, it will have the color blue</p>
</div>
<div>
<p>This paragraph is one of the two children of the div</p>
<p>This paragraph is one of the two children of its parent</p>
</div>
CSS：
p:only-child {
color: blue;
}
上面的例子选择了 <p> 元素，它是来自父元素的唯一子元素，在本例中是 <div>。