JavaScript key generators are not secure, and there have been a number of vulnerabilities and poor implementations which have resulted in wide spread losses. You should substitute using a JavaScript based website for a piece of good open source wallet software which uses properly secured random number generation, such as Bitcoin Core, Electrum, or Sparrow.
2) I have a PC with freshly installed version of Windows 10 - the PC has no Internet connection.
Use Linux instead.
3) Opened the html file on the non-Internet PC and created the wallets.
Once you've downloaded the wallet software you are going to use, you need to verify it against the developer's signatures before transferring it to your airgappd computer.