Considering a smart switch based on a power greedy WiFi ESP8266 (800mA spikes and 100mA on idle, @ 3.3V), there is the common situation to have the neutral wire missing from the wall switch box (left with 2 hot wires, maybe some earth wire that I don't want to consider in any way). Powering the electronics in the smart switch from the 2 hot wires will draw current through the light bulbs, making them either flash or light up (maybe except for some incandescent ones, but I can't rely on the light bulb type in any way; all usual types ranging from 1W to 100W, 110/230VAC should be considered).
At this point you might have already guessed the problem: what kind of smart switch design can bypass this issue? Some suggestions I read were to use an extra capacitor in parallel with the light bulb so that the current drawn by the switch won't get through it. Although I'm a beginner with electronics and I don't fully understand how exactly should that work, or how to scale the capacitor (maybe someone could explain me), I only wonder why didn't the LED/CFL bulbs manufacturers just include this feature in their products, since this seems to be an universal issue. Are there any reliable solutions to this problem?