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[Bug 16072] Margin collapsing: clarification needed as to where a collapsed margin manifests itself

From: <bugzilla@jessica.w3.org>
Date: Wed, 22 Feb 2012 08:41:20 +0000
To: public-css-bugzilla@w3.org
Message-Id: <E1S07lI-00086u-Sf@jessica.w3.org>

--- Comment #1 from Anton P <antonsforums@yahoo.co.uk> 2012-02-22 08:41:20 UTC ---
It was generally agreed in the F2F in Paris 2012
[http://lists.w3.org/Archives/Public/www-style/2012Feb/0529.html] that the
collapsed margin should manifest itself outside the parent's border box.  I
don't see a suitable place in 8.3.1 to mention this, and it's not really within
the remit of 8.3.1 to describe /how/ to use the resulting collapsed margin. 
Instead, I propose the following change to 9.4.1 (Block formatting contexts).


In 9.4.1, in the following paragraph:

  # In a block formatting context, boxes are laid out one after the
  # other, vertically, beginning at the top of a containing block. The 
  # vertical distance between two sibling boxes is determined by the
  # 'margin' properties. Vertical margins between adjacent block-level
  # boxes in a block formatting context collapse.

change the last sentence as follows:

  | Vertical margins between adjacent block-level boxes in a block
  | formatting context <ins>typically</ins> collapse<ins>, and the
  | collapsed margin manifests itself outside of the border areas of
  | all the boxes that have precisely one margin which participates in
  | the collapsed margin</ins>.

[Note that it's not true in general that a collapsed margin lump lies outside
of the (zero-height) border box of self-collapsing elements.  I added the word
"typically" because there are clearly various cases where margins don't

The resulting paragraph can then be rewritten with more accuracy in css3-box.

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Received on Wednesday, 22 February 2012 08:41:22 UTC

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