为什么要给父标签加上height：300px
<style type="text/css">
.mainbox{
width:960px;
background-color:#cff;
height:300px;
}
.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 class="leftbox">box2</div>
<div class="rightbox">box3</div>
</div>
