A strong password is one an attacker cannot guess faster than by trying every possibility. That comes down to how many possibilities there are, which is a function of length and character variety, and nothing else. Substituting a 3 for an E or adding an exclamation mark at the end does not help, because password-cracking tools have applied those rules for decades.
This generator offers two approaches. Random character strings maximise strength per character and are ideal for anything a password manager will remember on your behalf, which should be almost everything. Passphrases join several random words with a separator, which produces something you can actually type from memory or read aloud over a phone; they need to be longer to reach the same strength, but they are far easier to handle when a human has to.
Both modes draw from crypto.getRandomValues, the browser's cryptographically secure random source. This distinction matters more than it might seem: Math.random is fast but predictable, and given a few outputs an attacker can reconstruct its internal state and derive every other value it will produce. It should never generate anything security-relevant.
The estimated entropy shown alongside each password is the honest measure of strength: it tells you how many bits of randomness went in, which is the only number that reflects how hard the password is to guess. Anything above about 75 bits is comfortably beyond brute force with current hardware.