In CSP there's a [directive fallback list ](https://w3c.github.io/webappsec-csp/#directive-fallback-list) defined for various CSP directives. This spec should probably define one for the `monetization-src` directive. Something like: Return << "monetization-src", "default-src" >>. If it's not desirable to fallback to default-src I think this should be called out explicitly with a strong reasoning.