The concept of a HATEOAS API is also very simple: the API is defined by a communication protocol, 1 endpoint, and a series of well-defined media types. For a website, the protocol is HTTP, that 1 endpoint is /index.html, and the media types are text/html, application/javascript, image/jpeg, application/json and all of the others.
The purpose of this system is to allow the creation of clients and servers completely independently of each other, and to allow the protocols to evolve independently in subsets of clients and servers without losing interoperability. This is perfectly achieved on the web, to an almost incredible degree. There has never been, at least not in the last decades, a big where, say, Firefox can't correctly display pages severed by Microsoft IIS: every browser really works with every web server, and no browser or server dev even feels a great need to explicitly test against the others.
The point of HATEOAS is to inform the architecture of any system that requires numerous clients and servers to interoperate with little ability for direct cooperation; and where you also need the ability to evolve this interaction in the longer term with the same constraint of no direct cooperation. As the dissertation explains, HATEOAS was used to guide specific fixes to correct mistakes in the HTTP/1.0 standard that limited the ability to achieve this goal for the WWW.
Browsers can alter a webpage with your chosen CSS, interactively read webpages out loud to you, or, as is the case with all the new AI browsers, provide LLM powered "answers" about a page's contents. These are all recontextualizations made possible by the universal HATEOAS interface of HTML.
https://ics.uci.edu/~fielding/pubs/dissertation/net_arch_sty...
The server MUST be stateless, the client MAY be stateful. You can't get ETags and stuff like that without a stateful client.
https://ics.uci.edu/~fielding/pubs/dissertation/rest_arch_st...
> any concept that might be the target of an author's hypertext reference must fit within the definition of a resource
It's an optional constraint. It's valid for CSS, JavaScript and any kind of media type that is negotiable.
> resource: the intended conceptual target of a hypertext reference
> representation: HTML document, JPEG image
A resource is abstract. You always negotiate it, and receive a representation with a specific type. It's like an interface.
Therefore, `/style.css` is a resource. You can negotiate with clients if that resource is acceptable (using the Accept header).
"Presentation layer" is not even a concept for REST. You're trying to map framework-related ideas to REST, bumping into an impedance mismatch, and not realizing that the issue is in that mismatch, not REST itself.
REST is not responsible for people trying to make anemic APIs. They do it out of some sense of purity, but the demands do not come from HATEOAS. They come from other choices the designer made.
I'm realizing/remembering now that our internal working group's concept of HATEOAS was, apparently, much stricter to the point of being arguably divergent from Fielding's. For us "HATEOAS" became a flag in the ground for defining RESTful(ish) API schemas from which a user interface could be unambiguously derived and presented, in full with 100% functionality, with no HTML/CSS/JS, or at least only completely generic components and none specific to the particular schema.
"Schema" is also foreign to REST. That is also a requirement coming from somewhere else.
You're probably coming from a post-GraphQL generation. They introduced this idea of sharing a schema, and influenced a lot of people. That is not, however, a requirement for REST.
State is the important thing. It's in the name, right? Hypermedia as the engine of application state. Not application schema.
It's much simpler than it seems. I can give a common example of a mistake:
GET /account/12345/balance <- Stateless, good (an ID represents the resource, unambiguous URI for that thing)
GET /my/balance <- Stateful, bad (depends on application knowing who's logged in)
In the second example, the concept of resource is being corrupted. It means something from some users, and something to others, depending on state.
In the first example, the hypermedia drives the state. It's in the link (but it can be on form data, or negotiation, for example, as long as it is stateless).
There is a little bit more to it, and it goes beyond URI design, but that's the gist of it.
It's really simple and not that academical as it seems.
Fielding's work is more a historical formalisation where he derives this notion from first principles. He kind of proves that this is a great style for networking architectures. If you read it, you understand how it can be performant, scalable, fast, etc, by principle. Most of the dissertation is just that.
I guess someone interested would have to read the original work by Roy (who seems to have come up with the term) to find out which opinion is true
The only thing that would have made the web not conform to HATEOAS were if browsers had to have code that's specific to, say, google.com, or maybe to Apache servers. The only example of anything like this on the modern web is the special log in integrations that Microsoft and Google added for their own web properties - that is indeed a break of the HATEOAS paradigm.
Say I as a user want to read the latest news stories of the day in the NYT. I tell my browser to access the NYT website root address, and then it contacts the server and discovers all necessary information for achieving this task on its own. It may choose to present this information as a graphical web page, or as a stream of sound, all without knowing anything about the NYT web site a priori.