为什么.left里面得设置绝对定位，而不可以设置成float:left？
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>混合布局</title>
<style>
body{ margin:0; padding:0; font-size:30px; font-weight:bold}
div{ text-align:center; line-height:50px}
.top{ height:100px;background:#9CF}
.head,.main{ width:960px; margin:0 auto;}
.head{ height:100px; background:#F90}
.left{ width:220px; height:600px; background:#ccc; float:left;}
.right{ width:740px; height:600px;background:#FCC; float:right}
.r_sub_left{ width:540px; height:600px; background:#9C3; float:left}
.r_sub_right{ width:200px; height:600px; background:#9FC; float:left;}
.footer{ height:50px; background:#9F9; clear:both;}
</style>
</head>
<body>
<div class="top">
<div class="head">head</div>
</div>
<div class="main">
<div class="left">left</div>
<div class="right">
<div class="r_sub_left">sub_left
</div>
<div class=" r_sub_right">sub_right
</div>
</div>
</div>
<div class="footer">footer</div>
</body>
</html>
我这里设置了float:left，为什么中间红色间隙那么大而右边绿色那么小，跟题目要求的效果不一样，就是为什么得设置成绝对定位？
