When is the Scan Credit Card option available on iOS8 Safari?

Question

So Safari offers Scan Credit Card feature on iOS8 with some credit card forms.

My question is, how does Safari determine when to offer this option?<

Answer 1

For the expiration fields, based on Arnaud's answer, I found that the expiration fields would be recognized from cardExpirationYear and cardExpirationMonth being in the id attribute.

This worked when the year and month are regular text inputs with the appropriate IDs. The month is populated as a 2-digit number and the year as a 4-digit number.

In a quick test using tags, I found that it also populated the correct month and year.

01 02 ... 11 12 2014 2015 ... 2024 2025

I don't know what other values will work in the option tags.