Some of them don't do this though, which suggests it isn't a defining feature. The only "user input" in many cases is a click, which doesn't need to be collected and marshalled. The definition also includes "controls" no user input at all, such as when "onload" is the trigger, which I guess might be controversial, but the onclick controls are definitely controls and sometimes the click is the only input.

I think in the context of hypermedia there's a difference between the interface of the User Agent and affordances in the media. The "hyper" part of hypermedia refers to how resources are linked to and can influence each other. A resource can be linked to the resource that is responsible for creating it as in a Comment resource linking to a Reply resource. That reply resource needs a way to marshall whatever the elements of a Reply are (text, author, images, etc) into a new Comment Resource. That's missing.

Clicks on a page (and even the actual of typing or selecting an image) are part of the User Agent's interface: how the User Agent would interpret those hypermedia controls exposed by the Reply Reosurce into things the user can do. But the user Agent itself then needs to Marshall the result of user input into some kind of request. In HTTP this is a form and resulting POST request in urlencoded or multipart format.

A generic definition of this whole concept is needed.

You seem to have missed the point of the paper, since your comment here is presenting a definition of hypermedia as an alternative to the given definition of hypermedia controls. Defining hypermedia in general and discussing other aspects of it is not relevant to the scope of the paper. It honestly doesn't make any sense to complain that an academic paper specifically on hypermedia controls doesn't define and comprehensively discuss hypermedia as a whole, unless you have mistaken the purpose of the paper as being to define hypermedia as a whole. Your criticism of the definition of hypermedia and hypermedia systems might make more sense if it were directed at Carson Gross' book, Hypermedia Systems. Have you read it?

"four types of hypermedia controls found in HTML: links (anchors), forms, image tags and iframes"

"Examining the hypermedia control mechanic of links, forms, image tags and iframes in HTML, we see the following common functional pattern emerge: For each of these controls: (1) An event trigger (click, submit or load) (2) Causes a particular type of HTTP request issuance (3) That request is made to a resource location specified by a URL (4) The content of the HTTP response is then placed in the user agent viewport Within the context of HTML, this gives us the following functional definition of hypermedia controls as an element that incorporates this hypermedia control mechanic: Definition 4.1 (Hypermedia Control). A hypermedia control is an element that responds to an event trigger by issuing a type of request to a URL and placing the response at some position within the user agents viewport."

They mention the limitations of these existing hypermedia controls within HTML. In general, existing HTML controls don't allow the developer to choose any type of HTTP request to be sent from any type of HTML element and target any part of the page.

"in the absence of an extension mechanism hypermedia clients and servers must agree in advance on the types of hypermedia controls they will support. HTML includes a very powerful extension mechanism: JavaScript [47 ]. By taking advantage of this extension mechanism we are able to extend HTML and provide the generalized hypermedia controls outlined above to HTML authors."

"CONCLUSION In this paper we proposed an informal and then definition of the term "hypermedia control", derived from the implementations of four common such controls found in HTML: anchors, forms, im- ages and iframes. From this formal definition we then derived a generalization of the concept of hypermedia controls within the context of HTML. We then introduced htmx, a JavaScript library that implements these generalizations for HTML authors. We demonstrated two interactive patterns that can be implemented by HTML authors using generalized hypermedia controls. Finally, we introduced Hyperview, a mobile hypermedia that also implements the generalization we proposed. We demonstrated an interactive pattern achievable in Hyperview using this generalization, thus demonstrating that this generalization applies to hypermedia systems beyond HTML and the WWW."