  Fresnel reflection has been known for centuries to fundamentally impede
efficient transmittance across planar interfaces, especially at grazing
incidence. Herein, we present the generalized Huygens' condition (GHC) to
resolve such intricacies in metasurface (MS) designs and allow omnidirectional
transparency. Compared to common numerical antireflective-coating approaches,
our analytical framework yields surprisingly simple closed form conditions,
carefully leveraging the natural nonlocal mechanisms endowed in planar
electromagnetic structures. At the meta-atom level, we meet the GHC by
balancing traditional tangential susceptibilities of Huygens' MSs with their
unconventional normal counterparts; the latter facilitates the key requisite of
vanishing backscattering at the challenging grazing incidence scenario. At the
MS level, we sheerly utilize this central insight to engineer realistic
all-angle transparent printed-circuit-board (PCB) cascaded admittance sheets.
Thoroughly validated in simulation and experiment, this universal GHC
demonstrates a resourceful venue for practical implementation of advanced
nonlocal devices, e.g., flat optical components, optical analog computers, and
spaceplates.
