<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
div {
margin: 100px auto;
width: 0;
height: 0;
/* 透明色 */
border: 20px solid transparent;
/*想指向哪边，只需修改边框相对方向*/
border-top-color: red;
}
</style>
</head>
<body>
<div></div>
</body>
</html>
