Issue title: Introduction is aspirational, but misleading
 Issue body: The spec reads: 

> Buying things on the web, particularly on mobile, can be a frustrating experience for users. Every web site has its own flow and its own validation rules, and most require users to manually type in the same set of information over and over again. Likewise, it is difficult and time consuming for developers to create good checkout flows that support various payment schemes.

This implicitly suggests that proprietary, centralized, OS-privided payment methods provide a payment experience superior to what can by provided by the Web (maybe true, maybe not) - but a proprietary payment application can definitely create a bad user experience. Additionally, this goes counter to the goals of the "Payment-Request hander API" (aka "payment apps") - where the checkout flow is still provided via the Web.  

Additionally: 
> "Every web site has its own flow and its own validation rules"

"Every" is hyperbole. Many merchants use PayPal, Stripe, Shopify, etc. to provide a consistent flow and validation rules. 

I suggest dropping this paragraph entirely.  
 Comments: 
 Comment 0: Same with the assertion in the into that:

> In addition to better, more consistent user experiences, 

Which I also suggest dropping. 


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