Referrer policy's integration with stylesheets is spelled out in https://w3c.github.io/webappsec-referrer-policy/#integration-with-css, and tested in the files at https://github.com/w3c/web-platform-tests/tree/master/referrer-policy/css-integration. It would be lovely if WebKit would take a look at the implementation here, and align itself with the specification.
<rdar://problem/35275249>
*** Bug 185732 has been marked as a duplicate of this bug. ***