In the case of HTML pages, there is ongoing coordination. The server expects the client to have accessed something before that said "please input amount in doll

In the case of HTML pages, there is ongoing coordination. The server expects the client to have accessed something before that said "please input amount in dollars" or maybe "please input amount in cents". But if there is some new hypermedia format that supports currencies, it can be used instead of a string. What makes it proper hypermedia is that it isn't tightly-coupled with a specific API, it is as "universal" as HTML. "Nothing outside the API provided by hypermedia" means there is no tightly-coupling between server and client other than the protocol and hypermedia format. No schema, no out-of-band-information, no iPhone app... It's just like the browser works: your computer has no special anything for accessing Hacker News, only a fully decoupled client that understands HTTP(S)/HTML (the browser).