两个div浮动后使它水平垂直居中
使用相对定位position:relative（注意两个都要设置，下面有代码。）
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8"/>
<title>内联样式</title>
<style>
*{
margin:0;
padding:0;
}
#box{
width:800px;
height:800px;
background-color:lightgray;
}
#left{
width:200px;
height:700px;
background-color:lightblue;
float:left;
position:relative;
left:50px;
top:50px;
}
#right{
width:500px;
height:700px;
background-color:red;
float:left;
position:relative;
left:50px;
top:50px;
}
</style>
</head>
<body>
<div id="box">
<div id="left">nav</div>
<div id="right">section</div>
</div>
</body>
</html>