> What exactly is the criteria of being a "hypermedia capable format"?
One document must be able to link to another.
> Is there a predefined way to deal with it in XML?
Yes, Xlink:
> In the earlier example, I had assumed he did exactly what you would do in JSON: make up a field for the purpose.
The problem with this is that it's non-standard and nothing understands it that isn't custom-written for that particular API.
> Though, honestly, that would seem to be the purpose of the "Link:" header.
Yes and no. Although the Link header is useful, it's not part of the document, it's a transport layer header. REST is transport-layer neutral.
There are a number of specifications that address linking in JSON:
* JSON-LD http://www.w3.org/TR/json-ld/
* HAL https://tools.ietf.org/html/draft-kelly-json-hal-06
* Siren https://github.com/kevinswiber/siren
* Uber https://rawgit.com/uber-hypermedia/specification/master/uber-hypermedia.html
What exactly is the criteria of being a "hypermedia capable format"?
The linked website gives an example of how the response on a bank-account like API might include links to other actions; XML and JSON seem equally capable here, as they can both encode links.
He mentions on another page,
> Unfortunately, JSON is not a hypermedia format. There is no predefined way to deal with link discovery in JSON.
Is there a predefined way to deal with it in XML? In the earlier example, I had assumed he did exactly what you would do in JSON: make up a field for the purpose.
Though, honestly, that would seem to be the purpose of the "Link:" header.