Cross-origin error muting is possibly not interoperable

[domenic]

#2440 points out that cross-origin error muting is not interoperable. This is a tracking bug for writing good tests for it, adjusting the spec as appropriate, and filing implementation bugs on it.

It will probably wait until #958 is fixed since the infrastructure is currently pretty bonkers.

[domenic]

Merging into #10514 which outlines the problem more holistically.