[css3-exclusions] ordering exclusions by z-order

The dependency of exclusion order, and hence layout, on z ordering is still
a problem for us in Gecko. Currently nothing in CSS makes layout depend on
z-order, so we don't compute z-ordering until paint time. By adding this
new dependency you're making a fundamental change to the architecture of
CSS and will require us to make fundamental changes. Maybe there is no
alternative, but it sucks.

Also, CSS z-ordering is widely agreed to be super-complex and difficult to
understand, so making layout depend on it pulls a lot of complexity into
layout, and exclusions specifically. This may be unwise. One of the
complexities is that parts of the same element can be interleaved in
z-order with parts of other elements, so in fact section 3.4 is ambiguous.
It is not enough to say "exclusions follow the painting order". You
probably want to say something about exclusions being ordered according to
the order in which the borders of the exclusion elements would be painted.

When you combine this with CSS multi-column and regions, a block-level
exclusion element could break and have multiple border-boxes. This entire
spec seems to be written on the assumption that there will only be
one-border box. It might not be too hard to accumulate all border-boxes
into the exclusion region, but the spec needs to be updated to handle that.
But worse, with CSS regions, the border-boxes might not all be at the same
z-level; I think you could have two exclusion elements whose border-boxes
are interleaved in z-order. (E.g. when an element breaks across region
boundaries and the second region uses 'z-index' to land somewhere else in
the z-order.) If so, then ordering exclusion elements by z-order won't work.

Maybe you can rescue the situation by declaring that an exclusion is not an
element but something else, that an exclusion element can generate multiple
exclusions (corresponding to its multiple border-boxes), and these
exclusions can be ordered independently. (You'd need to audit the spec to
make sure that the distinction between "exclusion" and "exclusion element"
is correctly and clearly made everywhere.)

If you do that, you'll need to check carefully to make sure things work
when an element's first border-box is an exclusion box for its second
border-box and vice versa...

Rob
-- 
“You have heard that it was said, ‘Love your neighbor and hate your enemy.’
But I tell you, love your enemies and pray for those who persecute you,
that you may be children of your Father in heaven. ... If you love those
who love you, what reward will you get? Are not even the tax collectors
doing that? And if you greet only your own people, what are you doing more
than others?" [Matthew 5:43-47]