为什么实现不了等高布局
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>border实现等高布局</title>
<style type="text/css">
h3{
margin: 0;
padding: 0;
border: 0;
font-size: 18px;
}
.box{
width: 300px;
font: bold 18px/50px "Microsoft YaHei";
border-left: 300px solid #333;
background-color: #eee;
}
.left{
float: left;
width: 300px;
line-height: 50px;
margin-left: -300px;
color: #ccc;
}
</style>
</head>
<body>
<div>
<nav>
<h3>导航1</h3><h3>导航1</h3><h3>导航1</h3><h3>导航1</h3>
</nav>
<section>
<div>模块1</div>
</section>
</div>
</body>
</html>为什么结果是这个样子??

