<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
div {
width:0px;
height:0px;
border:5px solid #F60;
border-color: #f60 transparent transparent transparent;
border-top-width:20px;
}/*需要将宽高都设为零，边框四个角都有，留一个颜色，其余透明处理，变尖需要加border-top的宽度*/
</style>
</head>
<body>
<div></div>
</body>
</html>