CSS triangle generator
http://apps.eky.hk/css-triangle-generator/
注释:Internet Explorer 6(以及更早的版本)不支持属性值 "transparent"。
border-color http://www.w3school.com.cn/css/pr_border-color.asp
width: 0px;
height: 0px;
border-style: solid;
border-width: 0 100px 100px 100px;
border-color: transparent transparent #007bff transparent;