css3 flex实现div内容水平垂直居中的几种方法

/*1.方向*/
        /*默认方向（row正方向）*/
        /* flex-direction: row; */
        /*row反方向*/
        /* flex-direction: row-reverse; */
        /*columnz正方向*/
        /*flex-direction: column;*/
        /*columnz反方向*/
        /*flex-direction: column-reverse;*/

        /*2.换行*/
        /*flex-wrap: wrap;*/
        /*flex-wrap: wrap-reverse;*/

        /*3.direction+wrep组合*/
        /*flex-flow: row wrap-reverse;*/

        /*4.主轴对齐*/
        /*起点左对齐*/
        /*justify-content: flex-start;*/
        /*起点右对齐*/
        /*justify-content: flex-end;*/
        /*起点居中对齐*/
        /*justify-content: center;*/
        /*间隔左右分散*/
        /*justify-content: space-between;*/
        /*间隔内边距相等*/
        /*justify-content: space-around;*/
        /*间隔相等*/
        /*justify-content: space-evenly;*/
        
        /*5.交叉轴对齐 当flex-direction: row;修饰y轴, 当flex-direction: column;修饰x轴*/
        /*默认交叉轴对齐*/
        /*align-items: stretch;*/
        /*默认交叉轴居中*/
        /*align-items: center;*/
        /*默认交叉轴上对齐*/
        /*align-items: flex-start;*/
        /*默认交叉轴下对齐*/
        /*align-items: flex-end;*/
        /*默认交叉轴内容对齐*/
        /*align-items: baseline;*/

        /*6.多行交叉轴对齐*/
        /*align-content: stretch;*/
        /*多行交叉轴居中对齐*/
        /*align-content: center;*/
        /*多行交叉轴上对齐*/
        /*align-content: flex-start;*/
        /*多行交叉轴下对齐*/
        /*align-content: flex-end;*/
        /*多行交叉轴内边距相等*/
        /*align-content: space-around;*/
        /*多行交叉轴间隔左右分散*/
        /*align-content: space-between;*/
        /*多行交叉轴间隔相等*/
        /*align-content: space-evenly;*/

/*1.控制项目次序*/
        /*order: 2;*/
        /*2.按比例扩大空间，数越大空间越大，0值不扩大*/
        /*flex-grow: 2;*/
        /*3.按比例缩小空间，数越大空间越小，0值不压缩*/
        /*flex-shrink: 2;*/
        /*4.需要flex-direction配合使用，row=宽 column=高，并且优先级高于width hight, auto值 让位优先级*/
        /*flex-basis: 600px;*/
        /*5.flex=grow+shrink+basis*/
        /*grow =1 && shrink = 1 && basis = auto*/
        /*flex: auto;*/
        /*grow =0 && shrink = 1 && basis = auto*/
        /*flex: initial;*/
         /*grow =0 && shrink = 0 && basis = auto*/
        /*flex: none;*/
        /*6.align-self覆盖容器的align-items*/
        /*align-self: flex-start;*/