<link rel="license">
Points at the main content's license, where machines can read it.
It declares which license covers the document's main content — the photo, the text, the collection. Image search engines and open-license tools use this line to filter what can be reused, and it is how a Creative Commons image search finds yours.
The caveat is in the specification's letter: the license covers the main content, not necessarily the whole page — quotations and ads may have owners of their own. And the line declares, it does not contract: the human text of the license pointed at is what binds.
<link rel="license" href="https://creativecommons.org/licenses/by-sa/4.0/">