Ok, solved this. The validation for this part of the constant blending was a bit too strict https://www.khronos.org/registry/webgl/specs/1.0/#CONSTANT_COLOR_BL

Ok, solved this. The validation for this part of the constant blending was a bit too strict https://www.khronos.org/registry/webgl/specs/1.0/#CONSTANT_COLOR_BLEND https://www.khronos.org/registry/webgl/specs/1.0/#CONSTANT_C... . Somehow the conformance suite didn't catch this problem (go figure).