Re: [w3c/payment-handler] Edits regarding ordering based on 7 Dec WG teleconf (#242)

Hi @msporny and @kmealey,

> "the spec is now silent on ... Payment Instrument ordering"

I believe the specification has always been silent on payment instrument ordering within a payment application. We have endeavored to leave payment app internals up to payment app developers
and not discuss them in the Payment Handler API. We are developing a separate resource for payment app good practices:
 https://github.com/w3c/payment-request-info/wiki/PaymentAppPractice

While I agree with this part of one of the proposed sentences:
  " It is expected that User agents may take usage patterns, user experience optimizations such as reducing clicks..."

I don't think it is necessary to add to the specification because that's what user agent
developers do. We don't gain much by saying it. However, see a small proposal below.

Regarding this part:
 "It is expected that User agents may take ... merchant preferences into account."

Payment Request API does not define a mechanism for merchants to express 
preferences to the user agent. Therefore, I do not think we should mention 
merchant preferences in Payment Handler API. 

That is also one reason I do not support adding the algorithm. The algo defines
this order:
 * Manual user configuration
 * Usage patterns
 * Merchant preferences
 * User agent defaults

However:

1) Merchants have no way to express preferences through Payment Request API.
2) It is always the case that defaults are last for any similar algorithm.
3) It would not make sense for a user agent to override a manual
    configuration with an automatic configuration unless there were some need to 
    do so (e.g., security) in which case the user agent will do it anyway.

Therefore, without the "merchant preferences" part the remaining algorithm
the rest is essentially "by definition" and would not add materially to the specification.

I don't mind changing this sentence:
 "User experience details are left to implementers."

to:
 "User experience details (such as optimizations based on usage patterns) are left to implementers."

Ian

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