Generation happens locally in your browser. You can disconnect from the internet before running and you can close the window after generation (before reconnecting to the internet).
Source code is public and is available on github at the link below
Link to github
Authors Public Key: npub1qqqqqqp350slvdds7028l4yre5cuh8v38zseert25mxf7lkr2trsy0j2m8
This key miner is simple and does not use multithreading - it is not recommended to try and mine more than 5 characters. For mining keys with a larger difficulty, Rana is recommended.
Link to Rana
Your prefix must be hex, so you can only use the following characters: 0123456789abcdef