I have added a section about mapping of logical moderator keys to physical keys.
Note that the mapping below is only a suggestion and open for discussion. I have no horse in this race. What is important to me is that the usage of logical shortcuts are standardized in order to avoid interference with applications, and make it more intuitive for the user.
PHYSICAL KEY MAPPING
The logical OPT, CTRL and ALT must be mapped to physical keyboard keys, or key combinations where suitable keys are missing, such as IBM Model M that only have Ctrl and Alt keys.
For "PC" keyboard with a "Win" key, OPT should be mapped to "Win", CTRL should be mapped to Ctrl and ALT to Alt.
For keyboards without a suitable OPT alternative, an alternative key sequence must be used. [Suggestions? Could CTRL CTRL work as OPT? That would be CTRL CTRL CTRL for OPT CTRL]
Apple keyboards should map OPT to Command, CTRL to Ctrl and ALT to the Option key.