w3c认为margin叠加是合理的。不管是不是bug，但是这个问题真的存在。看下面这个例子：
< !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" />
</head><head>
<style type="text/css">
*{padding:0;margin:0;}
.box{background:#ccc; }
.content{background:#eee;margin: 20px auto;}
.an-box{ margin:50px auto;background:#999999;}
</style>
</head>
<body>
<div class="box">
<div class="content">margin: 20px auto</div>
<div class="content">margin: 20px auto</div>
</div>
<div class="box">
<div class="content">margin: 20px auto</div>
</div>
<div class="box">
<div class="content">margin: 20px auto</div>
</div>
<div class="an-box">margin: 50px auto</div>
</body>
</html>
解决这个问题的方法有不少，先来看看这个，在外层的容器中加：overflow:hidden;zoom:1；
< !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" />
</head><head>
<style type="text/css">
*{padding:0;margin:0;}
.box{background:#ccc; overflow:hidden;zoom:1 }
.content{background:#eee;margin: 20px auto; }
.an-box{ margin:50px auto;background:#999999;}
</style>
</head>
<body>
<div class="box">
<div class="content">这里两个还是存在叠加margin: 20px auto</div>
<div class="content">这里两个还是存在叠加margin: 20px auto</div>
</div>
<div class="box">
<div class="content">margin: 20px auto</div>
</div>
<div class="box">
<div class="content">margin: 20px auto</div>
</div>
<div class="an-box">margin: 50px auto</div>
</body>
</html>
还有外层的容器中加边框属性，在叠加元素加空块状元素等等。。还有就是用padding来取代margin
