Re: [browser-payment-api] Different card schemes have different mandatory field requirements (#9)

> I am happy to defer to the editors, but it would be nice if FPWD was able to accommodate China in its basic card model. That being said, I appreciate time is of the essence so maybe a commitment to resolve the issue in a future draft is enough.

+1 to the sentiment about China here.

Also, the variability of requirements across card schemes is something that writers of payment applications have had to accommodate for decades.  The brute force way of handling it is to write a specific application for a specific acquirer that is hard-coded to (e.g.) require expiry information.  

Proper layering of service (which knows what fields are required) and the application (browser?, which should be agnostic) has proven the only mitigating tactic I've seen so far.  "Service" need not (in this case) mean a web server - could be a customizable worker.  Please ignore any architecture references.

The choices are clear:  
1) make everything optional and let the PSP indicate that something is missing.
2) create a service that is configurable, or
3) bake into the application that "this kind of card requires PAN only."

That's our world, and it's a slippery slope.

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