CSS 用 border 跟 after 做三角形

利用 box model 與 pseudo selector 做 triangle

.selected::after {
    content: "";
    position: absolute;
    border-top: #66CBF9 10px solid;
    border-right: transparent 10px solid;
    border-bottom: transparent 10px solid;
    border-left: transparent 10px solid;
}


Reference: CSS-triangle-after-implementation