Can be default, unsafe-url, no-referrer-when-downgrade, no-referrer, origin, strict-origin-when-cross-origin, same-origin or strict-origin. See the Referrer-Policy spec for more details on the meaning of these values.
default
unsafe-url
no-referrer-when-downgrade
no-referrer
origin
strict-origin-when-cross-origin
same-origin
strict-origin
HTTP Referrer URL.