Update: Ratty in the comment noted that ⎋ is actually the ESC key button not the power button.
Update 2012: Added more symbols.
Unicode does define some other characters which are sort of Mac-specific.
|Character||Entity Code||Entity Name||Description|
||Command Key symbol|
||Option Key symbol|
||Shift Key symbol|
||ESC Key symbol|
||Delete / Backspace symbol|
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.