Flexbox ——弹性布局
Flexbox is a layout model for displaying items in a single dimension — as a row or as a column.
弹性布局是一种基于一维的行和列的布局模型。
The key feature of flexbox is the fact that items in a flex layout can grow and shrink.
弹性布局的关键特性就是item（项）可以放大和收缩。
Space can be assigned to the items themselves, or distributed between or around the items.
可以给item自己，item之间或周围，分配空间。
Flexbox also enables alignment of items on the main or cross axis, thus providing a high level of control over the size and alignment of a group of items.
弹性布局可以在主轴和交叉轴上排列items的尺寸、位置和方向。
