为什么我改变“.wrap-center”中margin上下的值，全部的div都跟着改变
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>不定宽块状元素水平居中</title>
<style type="text/css">
.container{
float:left;
position:relative;
left:50%
}
.container ul{
list-style:none;
margin:0;
padding:0;
position:relative;
right:50%;
}
.container li{float:left;display:inline;margin-right:8px;}
/*下面是代码任务区*/
.wrap-center{
width:195px;
background:#ccc;
margin:50px auto;
}
</style>
</head>
<body>
<div class="container">
<ul>
<li><a href="#">1</a></li>
<li><a href="#">2</a></li>
<li><a href="#">3</a></li>
</ul>
</div>
<!--下面是代码任务区-->
<div class="wrap">
<div class="wrap-center">我们来学习一下这种方法。</div>
</div>
</body>
</html>
