Privacy
The password is generated in your browser by
crypto.getRandomValues() - a cryptographic random number generator.
The page sends nothing to a server and stores nothing: reload it and the password
is gone, so copy it straight into your password manager.
About strength
Strength is measured as entropy:
length × log₂(alphabet size). From 72 bits is enough for ordinary
accounts, from 90 - with margin. Length beats "clever" symbols: a 20-character
password of lowercase letters and digits is stronger than a 10-character one
with special characters.
Tips
Use a unique password for every service; a password manager makes that easy - then you only remember one master password. Turn on two-factor authentication wherever possible - it protects you even if a password leaks.