HTML：<!DOCTYPE html PUBLIC>
<html>
<head>
<meta charset="UTF-8">
<link rel="stylesheet" type="text/css" href="float.css" >
<title></title>
</head>
<body>
<div id="header"></div>
<div id="mainbody">
<div class="box1">box1</div>
<div class="box2">box2</div>
<div class="box3">box3</div>
</div>
<div id="footer"></div>
</body>
</html>css:*{margin:0;padding:0}
li{list-style:none}
img{vertical-align:top;border:none}
#header
{
background-color: red;
height: 100px;
}
#mainbody
{
background-color: blue;
height: 800px;
width: 800px;
margin:0 auto;
}
#footer
{
background-color: black;
width: 800px;
height: 100px;
margin:0 auto;
}
.box1
{
height: 800px;
width: 300px;
float: left;
background-color: #B94FFF;
}
.box2
{
height: 800px;
width: 200px;
float: left;
background-color: #CCEEFF;
}
.box3
{
background-color: #00FFCC;
position: absolute;
top: 20px;
}box3绝对定位，无已定位祖先元素。但只设置了上方偏移量，按理说应该以浏览器为基准定位，可左边是与manbody的左边缘对齐的。如图：
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