Our designers at Apple often request a blending operation that is not supported in the CSS Compositing and Blending specification: plus-darker. You can find many examples and discussion of this mode if you Bing on the Google.
We’d like to add this mode to Level 2. Details are here: http://en.wikipedia.org/wiki/Blend_modes#Addition The math is pretty simple: R = min(0, (1 - D) + (1 - S)) (We should maybe consider adding plus-lighter as well, and subtractive modes based on designer feedback) Dean
