Crosbie, HTML5 is no longer about formatting. It’s about building web applications. That’s why it has stuff in it like geolocation and background threads. Now we can argue about whether payment is a critical core functionality for web applications or not, but your argument about published works and copyright doesn’t really apply.
Lucas, I have no idea what a payment API would really look like. I’m imagining something like this: I configure my browser so that Citibank is my preferred payment handler (or more likely a wizard script on the Citibank site does this for me). The payment API just exposes a function for passing a payee ID, amount, and callback to my preferred payment handler. How Citibank chooses to structure the interaction after that is up to them, but once the transaction is cleared the callback is called with some sort of token and my MP3 starts playing, or my ebook loads, or whatever.