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Re: Issue 158 proposed text

From: Anton Prowse <prowse@moonhenge.net>
Date: Sun, 04 Jul 2010 12:40:20 +0200
Message-ID: <4C306514.2030302@moonhenge.net>
To: www-style list <www-style@w3.org>
Tab Atkins Jr. wrote:
> On Thu, Jul 1, 2010 at 2:49 PM, Anton Prowse <prowse@moonhenge.net> wrote:
>> Tab Atkins Jr. wrote:
>>> So, then, current proposal for fixing issue 158.  As a reminder, it's
>>> meant to address the issue found at
>>> http://wiki.csswg.org/spec/css2.1#issue-158 and replace the text found
>>> at http://www.w3.org/TR/CSS2/visuren.html#flow-control .
>>>
>>> | Computing the clearance of an element on which 'clear' is set is
>>> | done by first determining the hypothetical position of the element's
>>> | top border edge within its parent block.  This position is
>>> | determined after the top margin of the element has been collapsed
>>> | with all appropriate adjoining margins per normal margin-collapse
>>> | rules, except that the clearing element's top margin is not allowed
>>> | to collapse with the clearing element's bottom margin.
>>> |
>>> | If this hypothetical position of the element's top border edge is
>>> | flush with or past the relevant floats, then no clearance is
>>> | applied. Otherwise, the top margin of the element no longer
>>> | collapses with preceding margins,
>> Doesn't this reintroduce the "preceding margins" ambiguity that was
>> potentially resolved in [1]?
> 
> No, as I'm no longer trying to make a special distinction about it
> only collapsing with certain types of margins.  It just collapses with
> everything that it would normally collapse with, except for the one
> exception I specify.  This exception properly excludes everything I
> want to exclude - in essence, it captures the notion of "preceding"
> more precisely, but without the chance of accidentally overruling
> normal margin-collapse rules.
> 

I can't tell if we're talking about the same thing!  What I meant was is
that you've reintroduced the term "preceding margins" in the prose
without defining what it means for a margin to be preceding.  (Eg, it's
still not clear to me from the 9.5.2 prose whether the first in-flow
child's top margin collapses with the clearing element's top margin once
clearance has been deemed necessary via the hypothetical position
calculation, although 8.3.1 very much indicates that it is: "When an
element's own margins collapse, and that element has had clearance
applied to it, [...]".)  If this term relates to your description of
which margins collapse when calculating the hypothetical position, then
it should be made explicit.


The current proposal also contains an ambiguity:

| Computing the clearance of an element on which 'clear' is set is
| done by first determining the hypothetical position of the element's
| top border edge within its parent block.  This position is
| determined after the top margin of the element has been collapsed
| with all appropriate adjoining margins per normal margin-collapse
| rules, except that the clearing element's top margin is not allowed
| to collapse with the clearing element's bottom margin.

You're deferring to the normal margin-collapsing rules, and yet you're
constraining how they're supposed to be applied; you can only do this is
you then redefine how the normal margin-collapsing rules work under this
constraint.

In particular, when you say that the clearing element's top margin is
not allowed to collapse with its bottom margin, I assume you mean that
the two margins must be regarded as non-adjacent (to use the terminology
of 8.3.1, which I recommend doing).  But what if the clearing element
contains an only child whose top and bottom margins collapse?  This
combined child margin would ordinarily be adjacent to the clearing
element's top and bottom margins and hence would combine with both.
However, since you're preventing the clearing element's top and bottom
margins from collapsing in your hypothetical position calculation, you
need to state how the combined child margin interacts with the parent
margins.

<div style="border: 1px solid">
	<div style="float:left; width:15px; height:15px"></div>
	<div style="margin: 10px 0">
		<div style="clear:both; margin: 10px 0 20px">
			<div style="margin: 15px 0"></div>
		</div>
	</div>
</div>

Does the 15px combined margin of the clearing element's child combine
with the clearing element's 10px top margin or its 20px bottom margin?

If it combines with the 20px bottom margin, then the clearing element's
10px top margin combines with its parent's 10px top margin, and the top
borders of these latter two elements are incident at 10px below the top
of the float, as per 8.3.1.6.1.

However, if it combines with the 10px top margin then the combined
margin is 15px, which combines with the clearing element's parent's 10px
margin giving a resulting 15px margin, which means that the border top
of the latter parent -- and hence of the clearing element itself -- is
15px below the top of the float, again as per 8.3.1.6.1.

In the latter scenario, the position of the top border of the clearing
element is flush with the bottom of the float, whereas in the former
scenario it's above the bottom of the float.  Hence the determination of
whether clearance is introduced is affected.


Note that the ambiguity is benign if the clearing element's margins
don't collapse with its parent's top margin (for instance, because the
parent has a non-zero top border width); in that case 8.3.1.6.2 applies
and so the clearing element's top border edge is the same as it would
have been if that element had a non-zero bottom border, which more or
less dictates that its child's combined 15px margin combines with its
top margin rather than its bottom margin for the purposes of calculating
the position of its top border.


>> [1] http://lists.w3.org/Archives/Public/www-style/2010Jun/0639.html

Cheers,
Anton Prowse
http://dev.moonhenge.net
Received on Sunday, 4 July 2010 10:41:56 GMT

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