最终效果
<!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>
body{
background-color: gray;
}
div{
width: 0px;
height: 0px;
/* border-top:10px solid red; */
border-bottom: 10px solid rgb(242, 247, 247);
border-left: 10px solid transparent;
border-right: 10px solid transparent;
}
</style>
</head>
<body>
<div></div>
</body>
</html>