From the latest rounds of spec discussion, and to match the explainer doc, Feature Policy should support an "allow='list of features'" attribute on iframes.
Practically, a list of features should translate into a set of declared whitelists, which should replace the declared whitelists in the parent document, for the named features, for the purpose of calculating the inherited policy for the nested frame.
Comment 1 by iclell...@chromium.org
, Jan 18 2017