It looks like Chrome (and probably other browsers) is optimising the
next-padding div out since it has no content. This means the margin
doesn't apply. Adding the nbsp, and making sure it takes up no space
itself, is enough to make the margin do the right thing.
I also tried using padding instead of margin, but this results in an
extra page at the end of the output (there'd need to be a way to stop
page-break and next-padding appearing at the end of the document). I also
tried setting a min-height on next-padding, but this had the same issue.
1f299ed73e