Clarify link element is not for general metadata expressions

[mattgarrish]

I just reported an issue about epubcheck's enforcing of the media-type attribute on link elements, but it made me consider that the method of identifying conformance in the EPUB Accessibility 1.0 specification actually violates the specification.

It only slips through the cracks because the identifier URLs are "external" to the container, so a media-type attribute isn't required.

I'm sure there are plenty of similar "linked metadata" statements you could make with link that would similarly violate the specification but would be undetectable by epubcheck.

The element is focused on linked resources in its requirements and explanations, but I wonder if we should also clearly note that it is not intended for these kinds of general expressions?

[iherman]

Can you clarify what is violated and how? I am a bit lost...

[mattgarrish]

The link element is for linking resources, or so is its sole focus in its definition. The EPUB Accessibility conformance statement isn't a "resource", it's a conformance identifier URL (that also happens to have a document at its destination, in this case, but that's not required of any identifier URL).

Up until 3.1, when we made an exception for server that might return different types of resources, it was required to specify a media-type attribute for any linked resource.

Epubcheck wasn't checking this, so when we created the Accessibility specification no one noticed that we were violating the requirement by not having a media-type attribute on the conformance links, which are:

<link rel="dcterms:conformsTo" href="http://www.idpf.org/epub/a11y/accessibility-20170105.html#wcag-aa"/>

Foruntately, the change in 3.1 makes this valid as far as epubcheck can tell. But I would not consider this a linked resource. It's a metadata conformance statement in the form of a URL.

So what I'm asking is should we add a note clarifying not to use link with these kinds of URLs? Or should we make clear that external links may not always resolve to resources?

[iherman]

Thanks for the clarification, @mattgarrish.

So what I'm asking is should we add a note clarifying not to use link with these kinds of URLs? Or should we make clear that external links may not always resolve to resources?

you also said, although just as a side-note:

that also happens to have a document at its destination, in this case

my Semantic Web past 😉 is not shocked at all that a URL is used as some an identifier, mainly when that URL also resolves to something on the Web. Which is the case here. The HTML <link> element may be used that way for metadata. We can add a note making it clear that a link does not always resolve to resources, but I believe the A11y spec does not violates the spec.

[mattgarrish]

We can add a note making it clear that a link does not always resolve to resources, but I believe the A11y spec does not violates the spec.

But the accessibility specification isn't a resource of the publication in any way. The URL is an identifier, but the wording of the link element says:

The metadata element MAY contain zero or more link elements, each of which identifies the location of a linked resource in its REQUIRED href attribute

If it said the link element identifies resources or makes metadata statements about the publication, I'd agree with you fully.

But everything about the link element is, quite honestly, geared at linked metadata records, with sort of a vague idea that you can associate other resources like author home pages.

I'm kind of worried about trying to rewrite the section at this stage, though. The new identifiers for the 1.1 Accessibility specification don't use link, so the problem is less pronounced now. And anyone using link to state other conformance identifiers won't get caught by epubcheck even when it's fixed, so we can quietly ignore the issue. I'm fine just deferring this and not getting into it this close to CR.

[iherman]

But the accessibility specification isn't a resource of the publication in any way. The URL is an identifier, but the wording of the link element says:

The metadata element MAY contain zero or more link elements, each of which identifies the location of a linked resource in its REQUIRED href attribute

The specification does not say that 'resource' must be a 'resource of the publication'. The term 'resource' is very general, at least in W3C land. After all, RDF = "Resource Description Framework" 😀

[iherman]

I'm kind of worried about trying to rewrite the section at this stage, though. [...] I'm fine just deferring this and not getting into it this close to CR.

+1 to that.