The URLs also don't need to exist as an HTML webpage. They just need to be unique (ideally globally unique). urn:uuid: is always a good option but using URLs un

The URLs also don't need to exist as an HTML webpage. They just need to be unique (ideally globally unique). urn:uuid: is always a good option but using URLs under a prefix you control is just an easy way to get a globally unique ID. (In fact one of the problems with live URL IDs is that people often accidentally change the ID if the like URL changes.)