Open website in your browser and save it on you computer, than disconnect internet and generate seed phrase safely and write it on paper.
You should download and verify the source code from Github rather than save the website from your browser, since you don't know if the website is actually running the published source code.
This includes bitaddress.org should be used on your offline device to avoid getting compromised, it should always be "prevention is better than cure"
Disconnecting your computer temporarily is completely insufficient in terms of security. There is nothing stopping malware already on your device from altering what Ian Coleman will display, or saving anything you enter to file and transmitting it to an attacker next time you connect to the internet. At a bare minimum, you should use a live and amnesic OS such as Tails as Loyce has described above. Better still is to use it only on a
permanently airgapped device which has been formatted and had a clean Linux OS installed.
I would also note that in general using any website which relies on Javascript is a bad idea for generating entropy. Ian Coleman (airgapped!) is useful for exploring a pre-existing seed phrase, checking different derivation paths, etc., but I would never suggest using it (or any other website, even if airgapped) for generating a wallet.