<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
*{
margin: 0;
padding: 0;
border: 0;
}
.box{
400px;
height: 200px;
border-radius: 30px;
border: 4px solid yellowgreen;
margin: 200px;
background: #fff;
position: relative;
z-index: 100;
}
第一种方式；
.box:after{
content: "";
40px;
height: 40px;
border: 4px solid yellowgreen;
border-bottom-color: transparent;
border-left-color: transparent;
position: absolute;
right: 40px;
top: -24px;
margin: auto;
background: #fff;
transform: rotateZ(-45deg);
}
第二种方式;
.triangle_border_down span{
display:block; 0; height:0; border-28px 28px 0; border-style:solid; border-color:#fc0 transparent transparent;/*黄 透明 透明 */ position:absolute; top:0px; left:0px;}第三种方式;
#box{
0px;
height:0px;
border-top: 20px solid red;
border-right:20px solid transparent;
border-bottom:20px solid transparent;
border-left:20px solid red;
}
</style>
</head>
<body>
<div class="box">
<span><span>
</div>
</body>
</html>