The note said a header's fo:margin-bottom just hangs into the body and does
not move it. Probed against soffice on one-page fixtures: a 20mm top margin
with a one-line 10pt header puts the body at 24.07mm with no gap and 29.07mm
with a 5mm one, three paragraphs at 37.21, a paragraph wrapping to two lines
at 33.14 (so rendered lines count, not paragraphs), and fo:min-height 20mm at
40.00 — the floor wins, the gap is not added on top. Padding counts too.