How do flex-units on inline elements interact with text-align:justify? E.g. <div style="text-align:justify;">abcd efgh <span style="padding:1*">Hello</span> ijkl mnop</div> where there's a soft line break after 'ijkl'. Do the spaces get the extra space, or does the flex get it?

How do vertical flex-units on inline elements interact with baseline alignment? E.g. if I have <img style="height:1*">, that's supposed to make the image height equal to the line-box height, but the image is supposed to be baseline-aligned so the line-box height has to be greater than the image height...

How do flex-units interact with collapsing margins? E.g. <div style="overflow:auto; height:500px;"> <div style="height:1*; margin:1*;">Hello</div> <div style="height:2*; margin:2*;">Kitty</div> </div> How does that work? Allocating flex height first and then collapsing margins means the child divs don't fill the container. But trying to collapse margins before resolving the flex heights seems crazy.

These are just a few of the questions that would have to be resolved. A great deal of the CSS spec would have to be carefully analyzed for interactions with flex-units, and many parts of the spec will become even more complicated.