Re: [w3ctag/design-reviews] Web Publications review (#344)

Still processing all of this feedback, but wanted to follow-up on `rel` stuff. Specifically...
>  I don't know of any specification that defines URLs to be usable as link rel values

[HTML5.2](https://www.w3.org/TR/html/links.html#sec-other-link-types) includes:
> The remaining values must be accepted as valid if they are absolute URLs containing US-ASCII characters only and rejected otherwise.

[RFC 8288](https://tools.ietf.org/html/rfc8288#section-2.1.2) refers to these as "Extension Relation Types":
>   Applications that don't wish to register a relation type can use an
>   extension relation type, which is a URI [RFC3986] that uniquely
>   identifies the relation type.

Consequently, to keep parity between `<link>` and `Link`, it has been a common practice in API design and endpoint discovery to use URLs as `rel` values--see [Discovery of Annotation Containers](https://www.w3.org/TR/annotation-protocol/#h-discovery-of-annotation-containers) for an example.

Cheers!
🎩 

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