Created attachment 121186 [details]
The flow-into property only works on block-level elements, and ::before and ::after pseudos can be styled with display:block. Styling the pseudos as block elements is not sufficient for flow-into to work on them, though.
// this does not display any namedFlow content in the pseudo element
If you add a content property, then the pseudo elements will throw that content away and work as regions:
// this works
I'm assuming there's an optimization that ignores pseudo elements if they have no content, but this path will need to check to see if flow-into is assigning content to the element.
This was fixed as part of a larger fix of regions & pseudo-elements in #90079.
*** This bug has been marked as a duplicate of bug 90079 ***