<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>左三角</title>
<style>
div {
width: 0;
height: 0;
border-style: solid;
border-width: 5px 0 5px 7px;
border-color: transparent transparent transparent red;
font-size: 0;
line-height: 0;
}
</style>
</head>
<body>
<div></div>
</body>
</html>
