Media documents should not block initial media request by CSP

What is the issue with the HTML Standard?

https://html.spec.whatwg.org/multipage/document-lifecycle.html#navigate-media invokes https://html.spec.whatwg.org/multipage/document-lifecycle.html#initialise-the-document-object which sets the policy container from navigationParams in https://html.spec.whatwg.org/multipage/document-lifecycle.html#shared-document-creation-infrastructure:concept-document-policy-container

However, all browsers do not set the CSP header for image documents: https://wpt.fyi/results/content-security-policy/generic/image-document-ignores-csp.html?label=master&label=experimental&product=chrome&product=edge&product=firefox&product=safari&product=servo&aligned Servo ran into this and added a test. Now I am implementing document loading infrastructure in Servo and discovered that this step was missing in our code compared to the specification.

Probably the specification needs updating to set navigationParams.policycontainer to null?

Compare the media navigation and shared document creation sections linked in the issue with the WPT image-document-ignores-csp test and the browser results. Trace how navigationParams.policycontainer is handled for image documents, then determine the specification change needed to match the observed behavior and Servo's test.