border三角形的3种方式

<!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">