Looks like it’s based on the Web Monetization W3C proposal.

webmonetization.org/docs/

Look neat, though I’m always a little hesitant when the thing involves crypto. while Interledger is the main driver of the peer-to-peer payments so far, there is nothing stopping a government or banking service from creating an OpenPayment compatible service, so long run there might be a lot of flexibility and less being tied to a specific cyrpto.