[css-compositing] group invariance

All,

we've been discussing my proposal internally. One of my coworkers brought
up an issue with group invariance in my proposal and I wanted to see if
people believe that this is an issue.

One of the principles of the Adobe imaging model is that adding a default
group should not change the appearance of the artwork. (Changing group's
parameters to isolate:true or knockout: true will change appearance)
In SVG which uses the <g> operator for grouping, this is also the case.

However, with my proposal, things will render differently.
For instance, this example:

<circle>
<circle alpha-compositing='dst-in'>

will render differently from:

<circle>
<g>

<circle alpha-compositing='dst-in'>

</g>


You can rewrite the second section as:

<circle>
<g alpha-compositing='src-over'> // src-over is the default

<circle alpha-compositing='dst-in'>

</g>

This code will do a 'dst-in' of the circle with an empty buffer. This empty
buffer is then composited over the first circle.

My question is:
a. Do people believe that group invariance is an important feature to have?
b. If so, do you want to see this fixed in the first spec or can we address
this in a later version?

Making alpha-compositing group invariant is quite complex. The math is
comparable to how 'enable-background' works.

Rik