<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.big_box{
width: 400px;
height: 260px;
overflow: hidden;
}
.box{
width: 400px;
height: 200px;
background: #0f0;
border-bottom: 20px solid transparent;
border-image: url(img/三角形.png);
border-image-width: 20px;
border-image-slice: 72;
border-image-repeat: repeat;
border-image-outset: 20px;
box-shadow: 0 0 30px #000;
}
</style>
</head>
<body>
<div class="big_box">
<div class="box"></div>
</div>
</body>
</html>
三角形png：
最终效果：