看我写的第一个代码
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.min.js">
</script>
</head>
<body>
<script>
$('p:nth-child(odd)').css("background","red");
$('p:nth-child(even)').css("background","green");
</script>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
</body>
</html>
始终没有效果，最后把script文件放在body最后，终于可以了。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.min.js">
</script>
</head>
<body>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<p>we can we can</p>
<script>
$('p:nth-child(odd)').css("background","red");
$('p:nth-child(even)').css("background","green");
</script>
</body>
</html>