content: ‘‘;
position: absolute;
border: 6px solid #5190ac;
border-color: #5190ac transparent transparent;
width: 0;
height: 0;
bottom: -12px;
left: 50%;
margin-left: -6px;
}
:after 写三角形 border
原文：http://www.cnblogs.com/ch-zaizai/p/6405802.html
