<!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>三角形</title>
<style>
div {
width: 0px;
height: 0px;
border: 100px solid rgba(255, 0, 0, 0);
border-top: 100px solid red;
}
</style>
</head>
<body>
<div></div>
</body>
</html>