In a link, if you put rel=license it: Indicates that the main content of the current document is covered by the copyright license described by the referenced document. Source: http://www.w3.org/wiki/HTML/Elements/link
So, for example, <a rel="license" href="https://creativecommons.org/licenses/by/4.0/">Copyrighted but you can use what's here as long as you credit me</a>
gives a human something to read and lets computers know that the rest of the page is licensed under the CC BY 4.0 license.