Unicode does define some other characters which are sort of Mac-specific.
⌘ - ⌘ - ⌘ - the Command Key symbol
⌥ - ⌥ - ⌥ - the Option Key symbol
⇧ - ⇧ - ⇧ - the Shift Key symbol
⎋ - ⎋ - ⎋ - the Power Button
Note: The Power Button and Shift Key are not Mac-specific. The power button is described as “broken circle with northwest arrow”, or an escape character from ISO 9995-7. The shift key is described as an “outline up-arrow”.
Even though these are defined in standard Unicode, there is no guarantee that they will exist in the font of the receiving browser, but they’re at least globally defined, so they’re fair game.