tr 元素定义表格行，th 元素定义表头，td 元素定义表格单元。
<html>
<head lang="en">
<meta charset="UTF-8">
<title>:nth-child()</title>
<style type="text/css">
tbody tr:nth-child(odd){
color:orange;
}
tbody > tr:nth-child(even){
color:green;
}
thead tr:nth-child(odd){
background:yellow;
}
</style>
</head>
<body>
<table border="1" cellspacing="0" width="200">
<thead>
<tr>
<th>序号</th>
<th>姓名</th>
<th>性别</th>
<th>年纪</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>张三</td>
<td>男</td>
<td>12</td>
</tr>
<tr>
<td>2</td>
<td>李四</td>
<td>女</td>
<td>11</td>
</tr>
<tr>
<td>3</td>
<td>赵五</td>
<td>男</td>
<td>12</td>
</tr>
<tr>
<td>4</td>
<td>王二</td>
<td>男</td>
<td>12</td>
</tr>
</tbody>
</table>
</body>
</html>