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Re: [CSS21] top margin edge - hence static position - is not well-defined

From: Anton Prowse <prowse@moonhenge.net>
Date: Sun, 27 Mar 2011 17:45:29 +0200
Message-ID: <4D8F5B99.9090903@moonhenge.net>
To: "www-style@w3.org" <www-style@w3.org>
CC: Alan Gresley <alan@css-class.com>
On 27/03/2011 17:32, Alan Gresley wrote:
> On 28/03/2011 12:31 AM, Anton Prowse wrote:
>> 10.6.4 concerning the height of absolutely positioned elements says:
>> # [...] the static position for 'top' is the distance from the top
>> # edge of the containing block to the top margin edge of [...]
>> But the vertical margin edges are not well-defined for elements whose
>> margins participate in margin collapsing. This is most noticeable for
>> self-collapsing elements, such as in the following test case in which
>> there is no sensible relationship between the border box position of the
>> "tricky" element (as determined by 8.3.1) and its 20px bottom margin:
>> <div style="border-bottom:1px solid"></div>
>> <div id="tricky" style="margin-bottom:20px">
>> <div style="margin-top:10px"></div>
>> </div>
>> <div style="border-top:1px solid"></div>
>> For the most part this glitch doesn't cause us any problem since the
>> spec avoids (albeit probably unconsciously) identifying individual
>> margin pieces from amongst a collapsed margin lump. However, 10.6.4
>> relies on identifying the top margin edge of a box which may very well
>> participate in margin collapsing.
>> What should we do to fix this? Where should the absolutely positioned
>> element be drawn in the following test case, and why?
>> <div style="border-bottom:1px solid; margin-bottom:20px"></div>
>> <div style="position:absolute; width:100px; height:100px; background:
>> yellow; margin-top:10px"></div>
>> <div style="border-top:1px solid; margin-top:20px"></div>
>> There's no interop on this issue, but that's because Fx and IE don't
>> perform tentative margin collapsing at all for abspos in order to
>> determine their static position.
> This is since the margins of elements with position absolute do not
> collapse [1] 8.3.1.

Alan, we're talking about the calculation of the _static position_ of 
absolutely positioned elements.

Anton Prowse
Received on Sunday, 27 March 2011 15:46:02 UTC

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