用css画实心三角形

.arrow-down:after{
content: '';
position: relative;
top: 12px;
left: 2px;
border-style: solid;
border-color: black transparent;
border-width: 6px 6px 0px 6px;
}

.arrow-up:after{
content: '';
position: relative;
bottom: 10px;
left: 2px;
border-style: solid;
border-color: black transparent;
border-width: 0 6px 6px 6px;
}