以下直接给出代码：

第一种：

<Html>
<head>
<style>
#a{
border-left:1px #333 solid;
border-right:1px #333 solid;
width:300px;
height:500px;
}
.b{
height:1px;
overflow:hidden;
border-left:1px #333 solid;
border-right:1px #333 solid;
}
</style>
</head>
<body>
<div>
<div class="b" style="margin-left:3px;width:294px;background:#333"></div>
<div class="b" style="margin-left:2px;width:296px;"></div>
<div class="b" style="margin-left:1px;width:298px;"></div>
<div id="a">
</div>
</div>
</body>
</Html>

第二种：

<html>
<head>
<title>css圆角效果</title>
<meta http-equiv="content-type" c>
<style type="text/css">
div.RoundedCorner{background: #9BD1FA}
b.rtop, b.rbottom{display:block;background: #FFF}
b.rtop b, b.rbottom b{display:block;height: 1px;overflow: hidden; background: #9BD1FA}
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}
</style>
</head>
<body>
<div class="RoundedCorner">
<b class="rtop"><b class="r1"></b><b class="r2"></b><b class="r3"></b><b class="r4"></b></b>
<br>无图片实现圆角框<br><br>
<b class="rbottom"><b class="r4"></b><b class="r3"></b><b class="r2"></b><b class="r1"></b></b>
</div>
</body>
</html>

第三种：

<style>
.divfloat {
margin:0 0 10px 0;
background: transparent;
}

.roundtop,.roundbottom{
display:block;
font-size:1px;
width: 100%;
background:transparent;
}

.roundb1,.roundb2,
.roundb3,.roundb4 {
display:block;
overflow:hidden;
}

.roundb1,.roundb2,.roundb3 {
height:1px;
border-left:1px solid #9B9B9B;
border-right:1px solid #9B9B9B;
background:#F3F3F3;
}

.roundb1 {margin:0 5px;background:#9B9B9B;}
.roundb2 {margin:0 3px;border-width:0 2px;}
.roundb3 {margin:0 2px;}
.roundb4 {
height:2px;
margin:0 1px;
border-left:1px solid #9B9B9B;
border-right:1px solid #9B9B9B;
background:#F3F3F3;
}

.roundboxcontent {
display:block;
padding:5px;
height:100%;
overflow:hidden;
border:0 solid #9B9B9B;
border-width:0 1px;
background:#F3F3F3;
}
</style>

<div class="divfloat">
<b class="roundtop">
<b class="roundb1"></b>
<b class="roundb2"></b>
<b class="roundb3"></b>
<b class="roundb4"></b>
</b>
<div class="roundboxcontent">
<div class="sidebarwrap">
这是圆角div内容部分...
</div>
</div>
<b class="roundbottom">
<b class="roundb4"></b>
<b class="roundb3"></b>
<b class="roundb2"></b>
<b class="roundb1"></b>
</b>
</div>