弹性容器（Flex Container） : 设置 display: flex 的父元素。
弹性容器的属性
** Flex-direction || Flex-wrap || Flex-flow || Justify-content || Align-items || Align-content **
Flex-direction
flex-direction: row || column || row-reverse || column-reverse
Flex-wrap
flex-wrap: wrap || no-wrap || wrap-reverse
Flex-flow
flex-direction 和 Flex-wrap的简写属性
Justify-content
justify-content: flex-start || flex-end || center || space-between || space-around
Align-items
flex-start || flex-end || center || stretch || baseline
Align-content
flex-start || flex-end || center || stretch
弹性项目（Flex Item） : 弹性容器内的子元素。
弹性项目的属性
** Order || Flex-grow || Flex-shrink || Flex-basis **
Order
数字值，默认为0
flex-grow 和 flex-shrink
默认情况下，flex-grow 属性被设置为 0，shrink 属性被为值为 1
Flex-basis
默认值是 flex-basis: auto
flex 简写
一次就设置好 flex-grow、flex-shrink 和 flex-basis 属性
如果只设置了 flex-grow 和 flex-shrink 值，flex-basis 会被默认设置为零。这被称为绝对弹性。
如果只设置 flex-basis，就得到一个相对弹性
(1) flex: 0 1 auto
(2) Flex: 0 0 auto
(3) Flex: 1 1 auto
(4) Flex: "正数"
Align-self
auto || flex-start || flex-end || center || baseline || stretch
