<!doctype html>
<html>
<head>
<title></title>
<meta charset="utf-8">
<style>
#d1{
width:0px;
height:0px;
border:20px solid transparent;
border-top-color:#0f0;
}
</style>
</head>
<body>
<div id="d1"></div>
</body>
</html>
网页显示效果：