<style>
.box{
width: 0;
height: 0;
border-top: 20px solid transparent;
border-left: 20px solid transparent;
border-right: 20px solid transparent;
border-bottom: 20px solid #ccc;
}
</style>
</head>
<body>
<div class="box">
</div>
保留一条边的颜色，其余的透明，盒子没有大小