If you have A nests B nests C nests D and the user decides to print B (not sure any user agent provides UI for that, but let's assume there is), the specification suggests events in B and C, not D. Is that really what we want?

Also, if you have A nests B1 and B2, what's the order of the events? Does it matter if either B1 or B2 is cross-site? (Is it quicker than postMessage()?)

Marking this as a good first issue as the first step needed here is writing tests for these scenarios, which is relatively straightforward, albeit a bit tedious perhaps. If there are any questions around getting started with tests, do let me know.