This is an umbrella bug for implementing Feature Policy (https://wicg.github.io/feature-policy/)
Other related bugs:
https://bugs.webkit.org/show_bug.cgi?id=167430 implemented a very limited version (just the "allow" attribute on <iframe>)
https://bugs.webkit.org/show_bug.cgi?id=183292 depends on this.
Web Platform Tests:
There is a list of potentially-supported features at https://github.com/WICG/feature-policy/blob/gh-pages/features.md, but those are each independent of the policy mechanism itself.
*** Bug 187816 has been marked as a duplicate of this bug. ***