Generate a password without a server seeing it

The reasoning is simple and rarely spelled out. A password's entire value is that nobody else has ever held it. When a remote service generates one, that value existed in a response before it reached you, and you have no way to audit what happened to it on the way or afterwards. Generated on your own machine, the only copy is the one you keep.

Length beats character class gymnastics. A long passphrase of unrelated words resists guessing better than a short string of symbols and is far easier to type on a phone, and most of the rules demanding one of each character type are older than the reasoning that replaced them. Choose length first, then satisfy whatever arbitrary composition rule the site insists on.

Put the result straight into a password manager. A generated password you intend to memorise will be shortened, reused or written somewhere worse within a week, which is how strong password generation quietly turns into weak password practice. The generator and the store are two halves of one habit.

The offline check is almost comically quick here. Disconnect the network and generate: values keep appearing, because the randomness comes from your own machine. A generator that stopped working offline would be telling you exactly where its values came from. Generate a few, close the page, and note that nothing about any of them was recorded anywhere you would have to take on trust.