Ah, so the key here is not generating them using a web client. Not that you aren't generating them using an application of any kind. Both of the tools mentioned here are applications. Applications that don't require internet to work, but applications nonetheless. You're not likely to find user-friendly means of doing this, since it is only really needed by those with a higher threat-model than the average joe. You can get something like this kind of offline generation of keys using an old Android phone with no internet connection. Just download Amber, turn on airplane mode, then create a new Nostr account in Amber. Once completed, you will be able to back up the private key as an nsec, an ncryptsec, or even as seed words, all offline. The problem you will then run into is, there is no way to USE your private key offline. It must be accessible by the application you are using to log into a Nostr client. Everything you do on Nostr requires a signature from that key. Good luck finding a convenient way to sign for every reaction, zap, and comment in an air-gapped way.