效果图如图所示：
image.png
关键代码：
css:
<style>
#demo{
width: 100px;
height: 30px;
background-color: #abc;
position: relative;
border:4px solid #333;
margin: 100px;
border-radius: 20px;
}
#demo:after,#demo:before{
border:solid transparent;
content:'';
height: 0;
width: 0;
position: absolute;
}
#demo:after{
border-width: 11px;
border-bottom-color: #abc;
left:39px;
top:-22px;
}
#demo:before{
border-width: 14px;
border-bottom-color: #333;
left: 36px;
top:-31px;
}
</style>
html:
<body>
<div id="demo">
</div>
</body>
