The spec says:

If there is no next fragmentation container and thus the remainder of the content after the break would be discarded, then the UA may visually replace the contents of the line, as it does for text-overflow.

in #390 (comment), Mats Palmgren said:

I think this "may" should be changed to a "must", or alternatively, the entire sentence be removed. I don't see any upsides to allowing UAs to have incompatible behavior for this common case.

The idea was that the behavior allowed by this "may" sentence was worse, and that we wanted browsers to do the full thing, but that it was more involved, and that doing it as a visual replacement was a tolerable approximation (only in the case of no subsequent fragmentainer).

If implementors are OK with going with the full thing, I'd be happy about deleting that.