纯css 画箭头

原理：

把容器的边框设置大一点，容器本身的长宽为0，只设置一边的颜色让其自然形成箭头

参考文章

http://yuiblog.com/blog/2010/11/22/css-quick-tip-css-arrows-and-shapes-without-markup/

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">

        * {
            padding: 0;
            margin: 0;
        }

        #demo {
            background-color: #333;
            height: 100px;
            position: relative;
            width: 100px;

        }

        #demo:after {
            content: ' ';
            height: 0;
            position: absolute;
            width: 0;
            border: 10px solid  transparent;
            border-top-color: #333;
            top: 100%;
            left: 50%;
            margin-left: -15px;

        }

    </style>

</head>
<body>
<div id="demo"></div>
</body>
</html>