I would posit that if rendering differences in font display are affecting A/B testing a workflow, the A/B testing apparatus has failed to control for confoundin

I would posit that if rendering differences in font display are affecting A/B testing a workflow, the A/B testing apparatus has failed to control for confounding factors and was probably invalid anyway. If OSX is known to render differently than Windows (and it's not just fonts), why are you comparing cohort A from Windows to cohort B from OSX? That is an analysis error.