css实现三角形

    height: 0;
    border-top: 2.667vw solid rgba(0,0,0,.6);
    border-left: 2.667vw solid transparent;
    border-right: 2.667vw solid transparent;
    border-bottom: 2.667vw solid transparent;
    background-color: transparent;
    transform: rotate(180deg);
    transform-origin: 50% 0;