css高度已知，左右定宽，中间自适应三栏布局：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
*{
margin: 0;
padding: 0;
}
.box>div{
height: 100px;
}
.box .left{
background-color: red;
width: 300px;
float: left;
}
.box .right{
background-color: blue;
width: 300px;
float: right;
}
.box .center{
text-align: center;
line-height: 100px;
color: #fff;
background-color: orange;
}
</style>
</head>
<body>
<div class="box">
<div class="left"></div>
<div class="right"></div>
<div class="center">第一种方法：浮动</div>
</div>
</body>
</html>
原文：http://www.cnblogs.com/cxzhijia/p/7395664.html