Support for rel attribute on iframe

[0xdevwrite]

Should the rel attribute be valid on the iframe element? The purpose of rel is to describe the relation of a linked resource to the current document. The iframe element is essentially a link that happens to be rendered within its parent document, thus it seems reasonable that it should support the rel attribute.

Currently rel is only valid on link, a, area, and form.

For context, in a REST API, I'd like to use a custom link relation on an iframe to indicate to the client what type of resource is being referenced by the iframe.

[brennanyoung]

This makes sense, but I'd go further and say that we need other ways of expressing semantic relationships more generally, especially within the document. There may be good reasons why rel is constrained to elements that are associated with http requests, and I'm fine with that, but not all content relationships are about http.

For example, if a heading 'belongs to' a list, then some kind of attribute could express this unambiguously. (Yes, I know that assistive tech has heuristics to figure this out, but why should ATs need to use heuristics? Why not be explicit?).

See:
https://discourse.wicg.io/t/proposal-general-relationship-attribute-for-non-link-semantic-relationships/6123