只设置了id=div1的左浮动的时候，div2的框为什么消失了
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>浮动模型</title>
<style type="text/css">
div{
border:2px red solid;
width:200px;
height:400px;
}
#div1{
float:left;
}
</style>
</head>
<body>
<div id="div1">栏目1</div>
<div id="div2">栏目2</div>
</body>
</html>
