The spec has changed on multiple occasions around whether flex items are definite sized or not, which is part of what of what's caused the lack of interop here.

The spec has changed on multiple occasions around whether flex items are definite sized or not, which is part of what of what's caused the lack of interop here. The problem here is specifically having a percentage sized child of a flex item (and Chrome and Safari don't match the current spec, as the flex item is indefinite sized in too many cases; Firefox doesn't either, as the flex item is definite sized in too many cases!); to make the flex item definite sized everywhere you can typically just add height: 0.