主容器：
    flex-direction: row | row-reverse | column | column-reverse;
    flex-wrap: nowrap | wrap | wrap-reverse;
    justify-content: flex-start | flex-end | center | space-between | space-around;
    align-items: flex-start | flex-end | center | baseline | stretch;
    align-content: flex-start | flex-end | center | space-between | space-around | stretch;
    子容器：

    • order 数值越小，排列越靠前
    • flex-grow定义放大比例，默认为0
    • flex-shrink项目的缩小比例
    • flex-basis定义了在分配多余空间之前，占据的主轴空间
    • flex属性是flex-grow, flex-shrink 和 flex-basis的简写，默认值为0 1 auto。后两个属性可选。