Re: [w3c/webpayments-payment-apps-api] Payment app identifier to manifest filename mapping (#48)

@jakearchibald 
> With "what we don't want" - I'm not sure you'd ever show this. The page would be able to detect "no matching payment methods" and show links to supported methods.

I'm not sure what you mean here. Do you suggest that the browser should know about every payment method in the world and suggest an appropriate payment app? For the most common ones like Visa, PayPal etc., this is feasible, but there's an incredible amount of existing payment methods around the world. I would prefer if the browser did not have to care about these individual payment apps. Having the merchant (who needs to care about the individual payment methods anyway) provide links to payment apps makes much more sense to me.

> "Installing the payment app" is problematic because (I assume) it'll involve a browser permission while you're showing two different origins on the screen.

The screens on that page are mockups, they are not supposed to be the final UI design. Security issues like this one can be easily addressed by minor tweaks, such as making the the payment dialog full screen in this case.

> I think we bypass a lot of the security complication if we allow the merchant to detect "no payment apps available", and they just show some links on a page, which are already well-understood by users in terms of who's offering the links, and who controls the content. Because it's a link, there'll be nothing new happening when the payment app requests permission.

Sure, but by letting the merchant query for this information, we provide a vector for fingerprinting. Judging by the ruckus around the [Battery Status API](https://www.theguardian.com/technology/2016/aug/02/battery-status-indicators-tracking-online), it feels like we should step carefully here. This might be an area where you have more expertise than me, though. Do you think, @jakearchibald, that there is a way for us to provide this "no payment apps available" information without exposing 1) information that can be used for fingerprinting, and 2) sensitive information about the user?

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