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

Even after using the autocomplete and ID methods described above, I had a label at the top of my page with the value Credit / Debit / Gift Card that prevented iOS from offering the Scan CC option. I ended up adding this label above my CC number field to trick iOS into offering the Scan CC option:

Card Number

Opacity of 0, or a font-size of 1pt prevents iOS from offering the option.