css三角形：css画箭头向下的三角形
.arrow {
position: absolute;
bottom: 0;
left: 50%;
transform: translateX(-50%);
width: 0;
height: 0;
border-style: solid;
border-width: 8px 5px 0 5px;
/* 上、左、下、右 */
border-color: #000 transparent transparent transparent;
}