css

<html xmlns="http://www.w3.org/1999/xhtml" lang="zh-CN">
<head>
<title>new</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="description" content="Akira.Juggling" />

<style>
    div{
        0px;
        height: 0px;

        border-top: 50px solid darkblue ;
        border-right: 50px solid red;
        border-bottom: 50px solid darkgreen;
        border-left: 50px solid blue;
    }
</style>

</head>
    <body>
        <div></div>
    </body>
</html>

效果