Well, as far as I'm concerned I have to trust that this form won't POST my passphrase to a server you own.
No personal offense intended, that is a genuine problem with your approach.
No personal offense intended, that is a genuine problem with your approach.
Then try your password.
Alternate solution: load the web page and then turn off your WiFi while you test it.
Once you've loaded all the 3-grams (~50MB of data), you can run entirely offline, close the page, and then go back online.