Browser fingerprinting can get creative at times, to say the least. eBay's WebSocket port scanner [1] and Reddit's abuse of DRM and JavaScript JIT exploits [2] from years ago are two examples of the kind of in-depth introspection you can perform completely in the background using nothing more than simple non-permission-gated APIs.

[1] https://blog.nem.ec/2020/05/24/ebay-port-scanning/

[2] https://iter.ca/post/reddit-whiteops/