the float ceiling.
As the float ceiling is relative to the border box, not the margin box,
of the parent flow, top margin must not be included.
This exposed a pre-existing bug whereby margins are discarded if a block
contains only floats and no content, affecting the tests
`float_intrinsic_height.html` and `margins_inside_floats_a.html`. As a
workaround, some invisible content has been added to the bodies of both
tests.
