Should a SXG document be considered SecureContext or not?

Say that liveness checks as described in https://github.com/WICG/webpackage/issues/376 are implemented and passing for a given SXG. It seems that the current document could be granted SecureContext.

Let's say that liveness checks are not passing.
It seems that the level of security is not as high, which would mean that SecureContext should not be granted. Such variation may actually break content so it might be better to not render content to the user, and render the content fetched from the actual web site instead.

A consequence is that while the liveness checks can be done in parallel to processing of the SXG (subresource loading, parsing...), the liveness checks should be validated before the first page rendering and any JavaScript execution.

For privacy/security purposes, even subresource loading should probably be postponed until these checks are done.

Start with the liveness checks described in WICG/webpackage#376 and the questions about SecureContext handling in this issue. A decision is needed for passing and failing checks, including whether to delay rendering, JavaScript execution, and subresource loading; done means these behaviors are specified.