三角形样例：
<!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>
div{
width:300px;
height:100px;
border:1px solid red;
position:relative;
}
div::after{
content:"";
position:absolute;
top:10px;
right:10px;
width:10px;
height:10px;
border-right:1px solid red;
border-bottom:1px solid red;
-webkit-transform: rotate(45deg);
transition:all 0.6s;
}
div:hover::after{
-webkit-transform: rotate(225deg);
}
</style>
</head>
<body>
<div>
</div>
</body>
</html>