W3C home > Mailing lists > Public > www-style@w3.org > July 2010

Re: Issue 158 proposed text

From: L. David Baron <dbaron@dbaron.org>
Date: Thu, 29 Jul 2010 18:31:58 -0700
To: "Tab Atkins Jr." <jackalmage@gmail.com>
Cc: Bruno Fassino <fassino@gmail.com>, www-style list <www-style@w3.org>
Message-ID: <20100730013158.GA27745@pickering.dbaron.org>
On Wednesday 2010-06-30 08:47 -0700, 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.

Saying that the top margin of an element can't collapse with its
bottom margin is ambiguous, since there could be margins in-between
the two, and you don't say at which point the collapsing breaks.
The simplest such case is:
  <div style="clear: left"><div style="margin-top: 100px"></div></div>

The current spec says that the collapsing should be done as though
the element has a nonzero top border width, and I think (based on
the rationale you gave, not on the proposed spec text) you're
proposing to change it to say that the collapsing should be done as
though the element had a nonzero bottom border width.

However, it's also not clear to me why you're proposing this change.
I'm actually not even sure that you intended it.

Either way, it should be more explicit at which boundary the
collapsing is inhibited.  (And it should also be done in a way that
doesn't contradict the current wording of the margin collapsing
section, in which "collapses with" is a transitive relation.)

> | 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, and the clearance is set to the
> | greater of:
> | * The amount necessary to place the top border edge of the block
> |   even with the bottom outer edge of the lowest float that is to be
> |   cleared.
> | * The amount necessary to place the top border edge of the block
> |   even with the previously computed hypothetical position of the top
> |   border edge of the element.  (Informative Note: This is necessary
> |   to handle the case where the float moves due to the element's top
> |   margin no longer collapsing with previous margins.)

The second bullet point here also seems like a change from the
current spec in the case of elements whose margins collapse with
each other and contain elements with margins, since it changes from
not including the child margins to including them.  At first glance
I think that change is probably a good one, but I'd be interested to
know if it's compatible with implementations.

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/
Received on Friday, 30 July 2010 01:32:30 GMT

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