Just in case it wasn't clear from all the replies here, using an unknown rel-attribute on links doesn't do anything, and since the default behavior is to use links normally, this just treats links ...

An HTML element is like a basic building block of an HTML webpage. An element can be extended with an attribute. In this case the Link element is modified by the Rel attribute.

With these attributes, you have greater control over how search engines identify certain links. Here's how to use them properly.

Previously it only said, "Google supports rel canonical link annotations as described in RFC 6596." So go check your canonical annotations if you are not sure ...