使用before、after偽類實現三角形的制作，不需要再為三角形增加不必要的DOM元素，影響閱讀。
<!DOCTYPE html>
<html>
<head>
<style>
div{
width: 100px;
height: 100px;
border: 1px solid #ccc;
}
div:before,
div:after{
content: "";
position: absolute;
top: 40px;
left: 100px;
width: 0px;
height: 0px;
border: 10px solid transparent;
border-left-color: rgb(255,255,255);
}
div:before{
top: 39px;
border: 11px solid transparent;
border-left-color: #ccc;
}
</style>
</head>
<body>
<div></div>
</body>
</html>