I wonder, because you can visit the site offline and still create passwords.
Is this site safe and good for use?
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Yes, this site is safe to use.
If you are still worried, run the application locally offline, as you mentioned, and take the proper precautions so the application can't "phone home".
From the GitHub page:
If you are security conscious you are of course encouraged to download the source code for this app and run it locally by simply opening the index.html file. If you run it from the web feel free to run it without a network connection to demonstrate nothing is sent or received from an upstream server once the initial page load is complete.
If you are extra paranoid, sift through the source code first.
If you are SUPER EXTRA PARANOID, take the advice, once again, found on the GitHub page:
If you want to be REALLY REALLY secure. Roll the dice with a flashlight under a black hood with a printout of the Diceware word list. No computers needed!
Not really kidding
The only issue I can think of is whether or not they are generating sufficiently "random numbers". That is for you to decide based on the method they use and the information they provide on the application page. IMHO, it's a safe enough method.