[css-compositing] blending in canvas

All,

the Compositing and Blending spec has a chapter on adding blending to the
Canvas 2d context [1].
Blending is currently defined by overloading the 'globalCompositeOperator'
[2] with additional values.

People have raised this as an issue because CSS split compositing and
blending into 2 properties and collapsing them for Canvas seems
inconsistent.
The main reason for this difference, is that Canvas is built upon Core
Graphics on the hood and this API set can't implement compositing and
blending separately. Because of this, splitting the properties can cause
combinations that are not implementable (ie source-atop + multiply).


What do people think?
Is it more important for the spec to be consistent, or to have a feature
that can be consistently implemented today?

Rik

1:
https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#canvascompositingandblending
2: http://www.w3.org/TR/2dcontext/#dom-context-2d-globalcompositeoperation