Created attachment 469029 [details]
See the attached testcase. Chrome and Opera have the background image (the orange bar) bottom positioned to the regular block, while Gecko has it start at the bottom of the floating block, which sticks outside the body element. (Gecko's behaviour is more desired, imo, but Opera and webkit are correct according to the spec, as far as I understand it. This is dangerous with people mostly just testing in Firefox.)
Tested in gecko 2.0b3 20100805.
I guess the question here is something that's actually a little ambiguous in the spec: whether the "root" block formatting context is established by the root element or by something above the root element. If it's established by the root element, then the root should use the height rules in 10.6.7 and this is correct. But I can't tell from 9.4.1 what establishes the root BFC.
The CSS WG discussed this at today's meeting (and at the end of yesterday's) and concluded Gecko is wrong.
FWIW, webkit seems to have since changed its behaviour, and is now doing the same as Gecko. (Since at least Chromium 14.)
(In reply to Sander from comment #4)
> FWIW, webkit seems to have since changed its behaviour, and is now doing the
> same as Gecko. (Since at least Chromium 14.)
Indeed, Safari 5.1.3 (534.53.10) and WebKit latest nightly (r107048) running on OS X 10.7.3 show the same behaviour as Gecko.
*** This bug has been marked as a duplicate of bug 333382 ***