三角代码

&:after {
position: absolute;
bottom: 0;
left: 50%;
content: "";
display: inline-block;
width: 0;
height: 0;
margin-left: -4px;
vertical-align: middle;
border-bottom: 4px solid;
border-right: 4px solid transparent;
border-left: 4px solid transparent;
}