Pick a time in one zone and read it in all the others at once, with the offsets shown and daylight saving handled by the browser's own database.
Yes, by the IANA database your browser ships. That is also why a zone is named as a place rather than an offset: Europe/London is the rule, and +00:00 is only what it happens to be today.
Because a wall-clock time near a daylight saving change is ambiguous. The first pass guesses an offset and the second corrects it with the offset that actually applies at that instant.
In UTC, with the local time in brackets. A message that says 09:00 without a zone is read wrong by everyone who is not sitting next to you.
Close enough for a meeting, not for a system. UTC is the atomic standard everything is defined against; GMT is a time zone that happens to sit on it in winter and moves to BST in summer. Write UTC in logs and APIs, and leave GMT to the shipping forecast.