https://css-tricks.com/

1. 使用CSS画出三角

box shadow with angle

content: " ";
border: 6px solid #fff;
border-left-color: transparent;
border-bottom-color: transparent;
display: block;
position: absolute;
bottom: -6px;
left: 50%;
-webkit-transform: translateX(-50%) rotate(135deg);
transform: translateX(-50%) rotate(135deg);
box-shadow: 2px -2px 2px rgba(1,1,1,.12);