[zarzavat]

I believe the technical term is “Schelling point”: something that people can decide on without communication.

Base64 is very close to the Schelling point of Base62 i.e. [A-Za-z0-9], requiring only a couple more additional decisions to be made: which two extra characters to add.

Unfortunately the original Base64 inexplicably got this wrong and chose + and / instead of the more sensible choice of - and _