高度宽度设置为0 设置边框尺寸.
<!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>
.box1{
width: 0;
height: 0;
border:50px solid transparent;
border-top-color: aqua;
}.
</style>
</head>
<body>
<div class="box1"></div>
</body>
</html>
