1.效果：
2.代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title> <style type="text/css">
* {
margin: 0;
padding: 0;
}
#bo {
/* 如果将宽高设为0，就可以形成一个有四个三角形的正方形 */
width: 100px;
height: 100px;
/* transparent：透明的 */
border: 100px solid transparent; border-top-color: deeppink;
border-left-color: red;
border-right-color: blue;
border-bottom-color: black;
}
#box {
width: 0px;
height: 0px; border: 100px solid transparent; border-top-color: deeppink;
border-left-color: deeppink;
/*border-right-color: deeppink;*/
/*border-bottom-color: deeppink;*/
}
</style>
</head>
<body>
<div id="bo"></div>
<br/>
<div id="box"></div>
</body>
</html>