效果图1:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
div {
width: 0;/*去掉width，效果让人意外*/
border: 30px solid transparent;/*把边框颜色设为透明*/
border-bottom: 30px solid red;/*下边的*/
border-top: 30px solid blue;/*上边的*/
border-right: 30px solid yellow;/*右边的*/
border-left: 30px green solid;/*左边的*/
}
</style>
</head>
<body>
<div></div>
</body>
</html>