Margin collapsing is a bit confusing, but if I understand correctly,
when collapsing a box's top margin with the vertically adjacent
sibling box above, we should "skip over" empty (0-height) boxes and
collapse their margin with *their* vertically adjacent sibling box
above, etc. Iterating until we hit the first non-empty in-flow block.
This pulls up the bottom part of the face on ACID2. :^)
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