This test case renders incorrectly:
The float is in between two collapsing margins, so it should get a self-collapsing anonymous parent block and be aligned with the top border edge of that block. That block is between collapsing margins (obviously). The top border edge of that block is defined to be the same as the top border edge of the parent block (since the block's top margin collapses with the parent block's). The parent block's top border edge is defined to be what it would be if that block had a non-zero top border edge.
See also bug 15062.
(In reply to comment #1)
> See also bug 15062.
Don't. See also bug 15105 instead.
Looks like this hasn't really moved on from: http://lists.w3.org/Archives/Public/www-style/2007Sep/0048.html
Everyone is broken per the spec, and it doesn't look as if the spec has changed.
Didn't mean to mark invalid!