Use rel="license" instead of rel="copyright"

[xfq]

References:

• https://html.spec.whatwg.org/multipage/semantics.html#linkTypes
• https://w3c.github.io/html/links.html#allowed-keywords-and-their-meanings
• https://github.com/validator/validator/issues/254

[joshbuchea]

Apologies @xfq, I accepted another pull request with your proposed change. Thanks for your contribution! And thank you for including references!

[xfq]

No problem :)