div+css无图片实现带背景圆角框

<!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>无图片实现圆角框</title>
<style type="text/css">
body{
padding:50px;
}
.container{
margin:0px auto;
width:600px;
border:0px solid #F00;
padding:10px;
}
/*利用背景颜色实现圆角框*/
div.RoundedCorner{background: #ccc}/*内容区背景色要和上下圆角边框背景色相同*/
b.rtop, b.rbottom{display:block;background: #fff}/*圆角处的背景色和主背景相同，这样才能出现圆角效果*/
b.rtop b, b.rbottom b{display:block;height: 1px;overflow: hidden; background: #ccc}
/*适当控制上下边框的边距形成台阶变化出现圆角*/
b.r1{margin: 0 5px}
b.r2{margin: 0 3px}
b.r3{margin: 0 2px}
b.rtop b.r4, b.rbottom b.r4{margin: 0 1px;height: 2px}
.content{
width:200px;
height:500px;
padding:10px;
}
</style>
</head>
 
 
<body>
<div class="container">
   <div class="RoundedCorner">
<b class="rtop"><b class="r1"></b><b class="r2"></b><b class="r3"></b><b class="r4"></b></b>
<div class="content">无图片实现圆角框</div>
<b class="rbottom"><b class="r4"></b><b class="r3"></b><b class="r2"></b><b class="r1"></b></b>
</div> 
</div>
</body>
</html>

补充：web前端 , HTML/CSS  ,