One way to handle complexity is to punt on it.

I think that's the right call. You don't want the website itself to handle the complexity. It makes much more sense for the browser to handle it.

That is the goal ? The spec's goal should be to keep it simple on the developer side and hiding/"punting" complexity to the platform/browser developer, is that not the sensible (and secure) way of doing this ?

Users would rather trust the browser to handle my money than each app developer ? We already do this with Apple and Google and other platform hosts. Both of them also build browsers, so it will not even be that difficult for either of them to build payment into the browser as they already have the complexity handled.

Microsoft probably has the stack as well if not at same scale, certainly at reasonable level given their consumer businesses collecting money directly from users.

Mozilla is the only one who will need to build it bottom up, however they have the most incentive to do it, given the revenue potential.

Yes they are shifting it to the platform.