Add support for merchant validation

[marcoscaceres]

closes #646

The following tasks have been completed:

• [x] Confirmed there are no ReSpec errors/warnings.
• [x] Added Web platform tests and https://github.com/web-platform-tests/wpt/pull/12423
• [ ] added MDN Docs (link)
• [ ] added MDN compat data

Implementation commitment:

• [x] Safari
• [ ] Chrome
• [ ] Firefox
• [ ] Edge (public signal)

Impact on Payment Handler spec?

---

Preview | Diff

[marcoscaceres]

Blocking on @aestes review/feedback. This is primarily to align with Apple Pay so really need someone from Apple to approve it.

[marcoscaceres]

Gecko tracking bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1474499

[marcoscaceres]

WebKit implementation: https://trac.webkit.org/changeset/226766

[marcoscaceres]

Added tests for constructor and for onmerchantvalidation attribute: https://github.com/web-platform-tests/wpt/pull/12424 and https://github.com/web-platform-tests/wpt/pull/12423

[marcoscaceres]

@aestes, wrote:

This looks good, although I was surprised to see that MerchantValidationEvent.validationURL can be initialized from the eventInitDict. Is that there for payment handlers' sake?

It's not handler related - but could be used there too, I guess: It used to be a requirement in the DOM Spec that there be a 1:1 relationship between initialization dictionary members and Event interface attributes.

@domenic, I see this has now changed in the inner event creation steps. It seems that requirement was loosened up a bit:

For each member → value in dictionary, if event has an attribute whose identifier is member, then initialize that attribute to value.

Or does the 1:1 relationship rule still hold?

@aestes, the .complete() still rely on .isTrusted - so IMO it would still be good to define how URLs are resolved when the MerchantValidationEvent is constructed (even if constructed unsafely in JS).

[marcoscaceres]

@domenic requesting prioritization on this when you have time, as currently I'm working on an implementation in Gecko - so would really appreciate your review.

[domenic]

The idea is still to have 1:1, but there are a few legacy exceptions we worked to accomodate with that clause.

Will do a full review tomorrow!

[marcoscaceres]

@domenic, hopefully better now :/

[marcoscaceres]

Third time lucky 🤞