没有使用position定位，效果为什么是一样的？
<!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>布局定位练习</title>
<style type="text/css">
*{margin:0; padding:0;}
#wrap{width:970px; margin:0 auto;}
#mainbody{ margin-top:15px;}
#left{float:left;width:110px; }
#mid{margin-left:15px;width:650px;border:1px solid #999;float:left;}
#right{
border:1px solid #999;
margin-left:790px;
}
</style>
</head>
<body>
<div id="wrap">
<div id="header"><img src="http://img1.sycdn.imooc.com//5369cd6e0001a15b09700088.jpg" width="970" height="88" /></div>
<div id="mainbody">
<div id="left"><img src="http://img1.sycdn.imooc.com//5369cd0e00011e3901090487.jpg" width="109" height="487" /></div>
<div id="mid"><img src="http://img1.sycdn.imooc.com//5369cd3c00013e9e06490439.jpg" width="649" height="439" /></div>
<div id="right"><p>欢迎使用金山软件出品的爱词霸，在线查询你英语词汇、句子释义</p><img src="http://img1.sycdn.imooc.com//5369cd540001d8e101770329.jpg" width="177" height="329" /></div>
</div>
</div>
</body>
</html>
