  We calculate the effective spatial dimension $d_\text{IR}$ of electron modes
at critical points of 3D Anderson models in various universality classes
(O,U,S,AIII). The results are equal within errors, and suggest the
super-universal value $d_\text{IR} \!=\! 2.665(3) \!\approx\! 8/3$. The
existence of such a unique marker may help identify natural processes driven by
Anderson localization, and provide new insight into the spatial geometry of
Anderson transitions. The recently introduced $d_\text{IR}$ is a measure-based
dimension of Minkowski/Hausdorff type, designed to characterize
probability-induced effective subsets.
