Consider following change (CSS 2.1):

9.4.1 Block formatting contexts

*Blocks in flex context,* floats, absolutely positioned elements, inline-blocks, table-cells, table-captions, and elements with 'overflow' other than 'visible' (except when that value has been propagated to the viewport) establish new block formatting contexts.

Where "block in flex context" is such a block that either has dimensional attributes (height,margin, etc.) defined in flex units or is contained in a block that has @flow attribute defined.

By the way, what happens when flex-units occur in contexts like tables and

HTML tables already have a concept of flex units. They are named there "relative length" units.

and position:fixed elements can be computed against their layout containers too.

Thus: div { position:absolute; width:25%; height:25%; left:1*; top:1*; right:1*; bottom:1*; }

will position such an element in the middle of abs. pos. container.

container for computation of left,top,right and bottom so flex values for these attributes are treated as undefined.

Adding flex-units support seems to me to be a very invasive change that

flexbox as a flexible entity will have exactly the same set of implications that needs to be resolved.

Flex element in David's proposal is exactly:

element { width:1*; } or element { height:1*; }

I cannot see of how it will be simpler to resolve.