Payment Request API
I'm skeptical. Browser vendors have too short attention span to work on difficult problems like this until the solution becomes good enough and widely adopted. One of the recent examples of this was Persona - an attempt to standardize login flow across browsers, also difficult problem, but  easier than payments. Persona was killed, not because the solution was not working or couldn't be further improved, but because it wasn't widely adopted after relatively short time (somewhere between 1-2 years if I recall correctly).