As part of https://github.com/web-platform-tests/wpt/pull/17606 I've found that that when A embeds B, B can navigate itself to a blob URL of B's creation. But if B hands that to A and A navigates B, it does not work when B is cross-origin. So it does seem to depend on the "source browsing context" (which really is a source document) in some manner.
This is basically to ensure we track https://github.com/whatwg/fetch/issues/666 in this repository as well.
As part of https://github.com/web-platform-tests/wpt/pull/17606 I've found that that when A embeds B, B can navigate itself to a blob URL of B's creation. But if B hands that to A and A navigates B, it does not work when B is cross-origin. So it does seem to depend on the "source browsing context" (which really is a source document) in some manner.