Cool seems to work well. My only complaint: there is not much information about who is behind this project and how encryption works...
Leading to: what happens if the website is not reachable or goes offline?
I did look at the sources tab, to check out why my laptop was using so much cpu during the decryption phase, and there are some argon2 webworkers that pop up, so some combo of that, and it clearly seems deterministic, since both my phone and laptop produce matching passes, when all params match.
I try to find out more about the project and its story. Thank you