Flex items can be generated by replaced elements, but I don’t find anything about this case in https://drafts.csswg.org/css-flexbox/ (except in a note about blockification of the display property).

https://drafts.csswg.org/css-flexbox/#cross-sizing says to "perform layout" without further details, but as far as I know there isn’t a definition of layout for replaced element that is independent of their context / layout mode. CSS 2 has separate sections for inline replaced elements, for block elements in normal flow, etc.

Should there be a normative sentence that https://drafts.csswg.org/css2/visudet.html#inline-replaced-width and https://drafts.csswg.org/css2/visudet.html#inline-replaced-height apply?

For comparison, https://drafts.csswg.org/css-grid/#intrinsic-sizes is more specific.