An extensive SEO article about Canonical Tags in HTML, by @BrockbankJames, was recommended in the @csaba_kissi newsletter.

It contains everything from intro stuff to implementation, best and bad practices, and more.

Canonical URLs: A Beginner's Guide to Canonical Tags

In the HTML standard definition, we'll find "canonical" just as an allowed value for the "rel" attribute of the link element. In the "Link types" section.

So, a canonical reference 😉, as stated in many places, is the RFC6596 via @ietf.