为什么结果变成了一条线？
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<style type="text/css">
.mainBox{
clear:both;
height:0;overflow:hidden;
width:960px;
border:1px solid red;
background-color:#CFF;
}
.leftBox{
width:740px;
height:300px;
border:1px solid red;
background-color:#C9F;
float:left;
}
.rightBox{
width:210px;
height:300px;
border:1px solid red;
background-color:#FCF;
float:right;
}
/*在此定义相应的类选择器，并根据要求设置相关CSS属性*/
</style>
</head>
<body>
<div class="mainBox">
<div class="leftBox"></div>
<div class="rightBox"></div>
</div>
