Today if a merchant requests shipping address or payer's contact information, the data collection is always handled by the browser. At last week's WPWG FTF in Foster City, there was interest among payment providers to be able to provide this information. Supporting this delegation can lead to better user experience because the payment app may have more accurate information than the browser.

We should support delegation for all fields that are currently requested via PaymentOptions: https://w3c.github.io/payment-request/#dom-paymentoptions