Flexbox containers are often used as organizational concepts, to ease re-flow on mobile etc. 

- I cannot use % for flexbox W or H

- I cannot omit W or H

This means that the content cannot dictate, and a Flexbox container cannot be used merely as an organizational concept to re-group and re-flow for responsive mobile devices.

Suggestion:

Flex box containers allow %, and they allow no specification of width or height.