Good question! 
There are two main reasons:
- First, there’s a slight delay between when the user types a value and when the password strength is calculated. It’s not huge, but it doesn’t feel completely smooth.
- Second, I don’t know exactly how the native strength score is calculated.
By using JavaScript, the calculation is instant, and with regex I can define the rules myself.
That said, using the native Password Strength state is totally fine too, it really depends on your needs!
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