column-span and margin-collapsing (was: [CSSWG] Minutes and Resolutions 2011-09-21)

fantasai minuted:

 >    ACTION howcome: Investigate Opera's precise behavior around consecutive
 >                    colspans, to see if they match fantasai's "they're just
 >                    wrapped in an anonymous BFC" proposal is accurate.

In Opera's implementation, elements with 'column-span: all' create a
BFC as per:

   http://www.w3.org/TR/css3-multicol/#column-span

Therefore, floats are not allowed to enter a spanner in Opera's
implementation.

Spanners collapse vertically, just like these BFC-creating elements
do:

   <div style="overflow: hidden; margin: 100px 0;"></div> 
   <div style="overflow: hidden; margin: 100px 0;"></div>

As such, we think collapsing is a natural fallout from the current
spec. Therefore, I'm not sure we need to describe anonymous BFCs in
the spec.

If we decide to *not* collapse spanners, we need to change the spec,
though.

-h&kon
              Håkon Wium Lie                          CTO °þe®ª
howcome@opera.com                  http://people.opera.com/howcome