Very nice work. I've done lots of work with credit card fields because I work for a large non-profit that depends on credit card donations. I might just use this in the next iteration of our form. Good idea doing the detection in realtime.

My only gripe would be that it doesn't innately explain which cards are acceptable... E.g., you might not accept Diners Club and Discover, but do want to accept Visa, MC, and Amex. Having little icons there make that more explainable. We did it like that on our contribution form: https://contribute.publicradio.org/contribute.php