css 三角形

li.downloadApp:hover::after {
content: '';
border: 8px solid white;
position: absolute;
top: 33px;
margin: auto;
left: 0;
right: 0;
width: 0;
border-top: none;
border-color: transparent transparent white transparent;
color: white;
}

width设置为0，是为了满足position:absolute;时候的恒等公式，不让width变为最大值；

left+margin-left+boder+padding-left+width+padding-right+border+margin-right+right=父元素内容区的宽度(开启绝对定位的情况下)

https://www.bilibili.com/video/BV1XJ411X7Ud?p=104