[CSS21] 8.3.1 Collapsing margins - editorial issues

I find the presentation order of 8.3.1 rather unhelpful.

It begins by vaguely defining "collapsing margins" without really
explaining what happens.

It then makes a note about horizontal vs vertical margins.

Next, it proceeds with an unordered list purportedly describing which
boxes participate in margin collapsing (but in fact mainly describes
which boxes /don't/ participate in margin collapsing), yet goes off on
tangents about /how/ margin collapsing works and how to calculate the
top border position of certain boxes.

It follows this with several short paragraphs that provide further
details about which boxes participate in margin collapsing that were not
mentioned in the list.


I propose reordering this section so that:

- it begins with a clear definition of what margin collapsing is and how
it works, as well as stating which boxes are ordinarily involved (list
item 1);

- it follows this with a list of boxes/elements which do not participate
(the existing list minus the tangential items);

- it follows this with the short paragraphs that provide further details
about which boxes participate in margin collapsing;

- it concludes with the discussion of how to calculate the top border
position of certain boxes.


I'd also like to re-raise an issue which I brought up in the past[1] but
which I think got lost in the discussion surrounding the clearance paradox:

The following sentence in the penultimate list item in 8.3.1 is redundant:

   # An element that has had clearance applied to it never collapses its
   # top margin with its parent block's bottom margin.

because lower down in 8.3.1 it is written that

   # When an element's own margins collapse, and that element has had
   # clearance applied to it, its top margin collapses with the adjoining
   # margins of subsequent siblings but that resulting margin does not
   # collapse with the bottom margin of the parent block.

and this covers a superset of cases. (An element could only collapse its
top margin with its parent block's bottom margin if its own margins
collapsed.)


[1] http://lists.w3.org/Archives/Public/www-style/2009May/0190.html

Cheers,
Anton Prowse
http://dev.moonhenge.net

Received on Sunday, 4 July 2010 11:51:53 UTC