设置在父元素上的属性
设置flex属性后，float,clear,vertical-align属性将失效
- flex-direction ：row | row-reverse | column | column-reverse ;
- flex-wrap : wrap | nowrap | wrap-reserve ;
- flex-flow : flex-direction || flex-wrap ;
- 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 ;
设置在item上的属性
- order : n ; 整数值
- flex-basis : 像素 | 百分比 | auto ; 项目占据的主轴空间，宽度会失效，需要和flex-grow和flex-shrink配合使用
- flex-grow : n ;
- flex-shrink : n ;
- flex : flex-grow || flex-shrink || flex-basis ; 默认0 1 auto
- align-items : auto | flex-start | flex-end | center | baseline | stretch ;
