transparent为透明
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>important属性介绍</title>
<style type="text/css">
div{
width:0;
height: 0;
border-top: 20px solid red;
border-left: 20px solid transparent;
border-right: 20px solid transparent;
}
</style>
</head>
<body>
<div>
</div>
</body>
</html>