<!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>
.box {
width: 100px;
height: 50px;
line-height: 50px;
background: lightskyblue;
text-indent: 20px;
}
/* 默认上三角形 */
span {
display: inline-block;
width: 0;
height: 0;
/* 上边框5px白色，其他边框隐形 */
border: 5px solid transparent;
border-top: 5px solid white;
position: relative;
/* 上三角形下移居中 */
top: 2.5px;
}
/* 鼠标滑过显示下三角形 */
.box:hover span {
border: 5px solid transparent;
/* 下边框显示 */
border-bottom: 5px solid white;
position: relative;
/* 下三角形上移居中 */
top: -2.5px;
}
</style>
</head>
<body>
<div class="box">
导航
<span></span>
</div>
</body>
</html>
默认上三角，鼠标滑过显示下三角
