:nth-child(n)：父元素的子元素的第n个
:nth-of-type(n)：父元素的 某类 子元素的第n个
注：n从0起，元素index从1起
nth-child(n)和nth-of-type(n)例：
<head><style>body :nth-child(2) {height: 30px;background: blue;color: #fff;}</style>
</head>
<body><p>1 - p1</p><div>2 - div1</div><p>3 - p2</p><div>4 - div2</div>
</body>
替换 :nth-child 为 :nth-of-type
<style>body :nth-of-type(2) {height: 30px;background: blue;color: #fff;}
</style>
