Reported by Øyvind Stenhaug
9.4.1 (Block formatting contexts) says that block-level boxes in a block formatting context are laid out one after the other, vertically, beginning at the top of a containing block.
But the position of a block-level element's top content edge isn't clearly defined: 8.3.1 fails to explain where a collapsed margin sits in relation to the border edge positions of the elements involved (for example in the case where an element's top margin collapses with the top margin of its first in-flow child).
The old text of 10.6.3 was faulty but it did try to ensure that this gap in the spec was filled for the cases it discussed. However, that section was then changed as per the resolution of Issue 225 [http://wiki.csswg.org/spec/css2.1#issue-225], which meant that this gap became more exposed. On the other hand, 10.6.7 never took account of this gap.
Conversation begins: http://lists.w3.org/Archives/Public/www-style/2011Apr/0132.html (first part)
Bug description: http://lists.w3.org/Archives/Public/www-style/2011Apr/0217.html