思路:
1.写一个div
2.将正方形的border加粗
3.隐藏其余三边
上代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.sanjiaoxing{
height:0px;
width: 0px;
border-top:50px solid red ;
border-right:50px solid transparent ;
border-bottom:50px solid transparent ;
border-left:50px solid transparent ;
}
</style>
</head>
<body>
<div class="sanjiaoxing"></div>
</body>
</html>