flex布局详解

弹性布局，非常容易实现垂直居中

父容器属性

• display:flex;
• flex-description: row | column | row-reverse | column-reverse;
• flex-flow: wrap | nowarp;
• flex:[ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ]
  默认：1 0 auto;