左右浮动 三列布局
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
.left{float: left;width:200px;height:500px;background-color:red}
.right{float: right;width:200px;height:500px;background-color:red}
.middle{height:500px;margin:0 210px;background-color:#000}
</style>
</head>
<body>
<div class="main">
<div class="left"></div>
<div class="right"></div>
<div class="middle">111222</div>
</div>
</body>
</html>