Currently the AMP documentation requires a link rel="canonical" in the head of the document. But there are two linking mechanisms that can accept rel values: link and a.

How about allowing rel="canonical" to sit on a elements as well?

Here's an example of an AMP HTML document with both: https://adactio.com/journal/9646/amp

That page currently fails validation because it has rel="canonical" on an a element that links back to the regular HTML version.