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Fix the duty cycle to a particular slice of the phase of AC voltage (e.g. the first 0-pi/2) and you accomplish the same thing much more practically, at least within a single power network.

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It's going to be a bit trickier than that, because power is usually distributed as 3-phase, not single-phase.

So that's 3 different offsets, depending on which phase any given light is wired into.

Plus that's probably going to be slow enough to cause visible flicker.

Have it trigger on a voltage threshold. 3 phase means you're hitting 0V 6x per cycle so that'd bring the pwm up to 720Hz.

Not a power engineer, but can't most of the power conditioning devices used in buildings mess with the phase of the incoming AC signal? GPS clock synchronization is guaranteed to be globally accurate.

They may cause harmonics, but the underlying sine wave is solid like a rock (well, at least concerning the ENTSO-E over here in Europe, other grids may be more fragile).

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