Most consumer electronic devices these days use switched-mode power supplies, which typically specify they can take input power between 100–240 V, at frequencies of 50–60 Hz AC. All my typical travel devices (phone, camera, laptop chargers) have these ranges, and with a universal plug/adapter (i.e. just a physical adaptor, no transformer) you can then plug them in pretty much anywhere.
But are there any countries with mains power outside this range? Is there anywhere that I need to be careful about or avoid using such devices? Or does the range completely cover all countries' mains power supplies?
I would hope (and am assuming) that the range is designed to cover everywhere in the world with mains power, but can imagine if one or two small countries were outside this range (e.g. 50V or 300V, or 100 Hz), it may not be worth the effort of all devices supporting them.