<!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>
<style type="text/css">
/*在此定义相应的类选择器，并根据要求设置相关CSS属性*/
.mainBox{
width:960px;
height:300px;
background-color:#CFF;
clear:both;
}
.leftBox{
width:740px;
height:300px;
background-color:#C9F;
float:left;
}
.rightBox{
width:210px;
height:300px;
background-color:#FCF;
float:right;
}
}
</style>
</head>
<body>
<div class='mainBox'></div>
<div class='leftBox'></div>
<div class='rightBox'></div>
<!--在此添加相应的div标签-->
</body>
</html>
<div class='mainBox'></div>应该包含<div class='leftBox'></div><div class='rightBox'></div>这俩，你给写成并列了