You can actually just do this on any website. When making an account, just type password into the password field. The best part is you can do the exact same thing at login, so you never have to memorize the super secure password they generate.
Though there's barely any difference in strength. It looks like it has upper and lower case for 'low', and 'high' adds numbers and the 10 symbols on the number keys. That's 5.7 bits per character vs. 6.2 bits per character. The 'high' strength passwords are only 8% stronger.