<!DOCTYPE html>
<html>
<head>
<title>第四节</title>
<link rel="stylesheet" type="text/css" href="disanjie.css">
</head>
<body>
<div></div>
</body>
</html>
div{
width: 0px; //注意width和height为0；
height: 0px;
border: 200px solid black;
border-left-color: yellow;
border-bottom-color: red;
border-right-color: blue;
border-top-color: green;
}
注释：如果width和height不为0，那么中间会有一个width和height相对应的块。
div{
width: 100px;
height: 100px;
border: 200px solid black;
}
div{
width: 0px;
height: 0px;
border: 200px solid transparent; //transparent透明
border-left-color: yellow;
border-bottom-color: yellow;
}
