Intent to implement per element referrer attributes

[Franziskus Kiefer]

Hi all

We intend to implement the referrer policy delivery method via attribute as
specified in the referrer policy spec (editor's draft [1]) to allow per
element referrer policies for <a>, <area>, <img>, and <iframe> tags. The
attribute is enabled via the pref network.http.enablePerElementReferrer and
will be initially disabled by default. Referrer policies allow to specify
which referer [sic] header is sent when performing the request (there are
currently three policies specified in [1]). The referrer attribute allows
developers to override the document's referrer policy (set in meta tag or
via CSP) on a per element basis.

[1]
https://w3c.github.io/webappsec/specs/referrer-policy/#referrer-policy-delivery-referrer-attribute

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=999754

Link to standard:
https://w3c.github.io/webappsec/specs/referrer-policy/#referrer-policy-delivery-referrer-attribute

Platform coverage: all