[css-compositing] Some alterations to the spec

All,

I had a talk with Simon Fraser at the webkit developers meeting last week.

After a very productive talk, we decided the following:
1. background-blend-mode should only blend between the background color and
background images.
The reason is that one or more of an element's (or one of its parents)
prior siblings could be in a different layer, so their color would not be
available.
By 'isolating' the blending of background images we have a known
environment.

2. the <area> property of mix-blend-mode suffers from the same problem so
we will remove it.

3. mix-blend-mode on an element should still work. It's imperative that it
establishes a stacking context.

One open question is still what establishes the element's backdrop. Should
it be anything that establishes a stacking context or my previous proposal
that lists a set of 'group' establishing css properties.?
Simon, please let me know if this jives with your recollection of our talk.

Sylvain is going to help me edit the spec as well.
We will split off the part of the spec that describes the math of
compositing and blending and will add in a normative reference to it.

Let me know if this proposal sounds more reasonable!
The feature that I'm cutting will move to the next level of the spec. (They
are possible but require a lot more work and I prefer little steps :-))

Rik