flex子元素可以直接设置z-index:

Flex items paint exactly the same as inline blocks [CSS2], except that order-modified document order is 
used in place of raw document order, and z-index values other than auto create a stacking context even 
if position is static (behaving exactly as if position were relative).


https://drafts.csswg.org/css-flexbox-1/#painting
https://www.sitepoint.com/quick-tip-how-z-index-and-auto-margins-work-in-flexbox/  //解析