Comment by JJJollyjim

Chrome doesn't allow it - local network services have to opt-in to being fetchable from public sites (https://github.com/WICG/private-network-access), although they're replacing it with a user-permission-based approach (https://github.com/WICG/local-network-access).

(There is some language online suggesting PNA has not actually shipped, but I experienced it myself in stable Chrome several years ago, so I am unsure of the current state).

Firefox doesn't implement either approach -- I assume this is indicative of their lack of development resources.