css能实现这样的内凹圆角吗？如何实现？具体代码是！！
比较笨的方式，截图以及代码具体如下。
<html>
<head>
<meta charset="utf-8"/>
<title></title>
<style type="text/css">
.cro{
width: 100px;
height: 100px;
border: 1px solid #58C4E6;
position: relative;
}
.cro_left_top,.cro_right_top,.cro_left_bottom,.cro_right_bottom{
position: absolute;
width: 20px;
height: 20px;
border: 1px solid #fff;
z-index: 1;
background: #fff;
}
.cro_left_top{
top: -1px;
left: -1px;
border-radius:0px 0px 20px 0px;
border-bottom: 1px solid #58C4E6;
border-right: 1px solid #58C4E6;
}
.cro_right_top{
top: -1px;
right: -1px;
border-radius:0px 0px 0px 20px;
border-bottom: 1px solid #58C4E6;
border-left: 1px solid #58C4E6;
}
.cro_left_bottom{
left: -1px;
bottom: -1px;
border-radius:0px 20px 0px 0px;
border-top: 1px solid #58C4E6;
border-right: 1px solid #58C4E6;
}
.cro_right_bottom{
right: -1px;
bottom: -1px;
border-radius:20px 0px 0px 0px;
border-top: 1px solid #58C4E6;
border-left: 1px solid #58C4E6;
}
</style>
</head>
<body>
<div class="cro">
<div class="cro_left_top"></div>
<div class="cro_right_top"></div>
<div class="cro_left_bottom"></div>
<div class="cro_right_bottom"></div>
</div>
</body>
</html>
