Playing around with some of these I clicked on one button and discovered that Chrome has a built in payment system -- the button opened up a dialog with all my saved information. Neat.
https://www.html5rocks.com/en/tutorials/forms/requestautocom...
I think this API ended up being deprecated from lack of use (not surprising as no one used it and is amazed it existed), but it may have paved the way for https://developer.mozilla.org/en-US/docs/Web/API/Payment_Req....
Genuinely surprised that this didn't catch on, it was amazing.
RequestAutocomplete had some weird quirks that made adoption slow and full of friction. You're right to say that PaymentRequest is a fixed RequestAutocomplete in most regards, and it's amazing (I'm biased though, I'm one of the people implementing it on desktop Chrome :) ). It's being implemented by all major browsers too!