[elliotanderson]

The GP is referring to doing client side Luhn validation on the credit card number, giving the user feedback if they mistype a number.

It's a technique I use, along with only allowing digits and doing some format masking (i.e. spaces or dashes to split the number into 4 digit blocks) to make the number more readible

[adgar]

Actually, the GP is talking about somehow building a startup based on... form widgets. He specifically said a "(subscription) service" - which would invariably require server-side support for these apparently startup-backed form fields.