Zero width combining marks

There is no reason to do so. I give all my accents a left and right side bearing of 20. The spacing gets corrected on export automatically.

Small note: In FontLab (that’s FL 5.04 on Windows), the diacritics with zero width and drawn left of the origin are terribly slow to edit. No idea why but that is what it is like. So, that would be a reason against the practice.