What is the issue with the HTML Standard?

There is a fringe case (#10404 (comment)) that is underspecified -- which global, if any, receives an error event the job passed to the HostEnqueuePromiseJob hook completes abruptly, but the realm is null.

This can be achieved (in the NewPromiseReactionJob case) by subclassing Promise to inject a custom resolve function which succeeds the first time (allowing a promise of this type to be created) and then throws the next time, inside then(null), which leads to such an abrupt completion with no realm. If these odd things are spread across realms (e.g., with same-origin frames) then it's not clear which global the error event ought to go to.

Major browsers to different things in this extreme edge case. It's unlikely to affect web content in the wild, but would be nice to nail down anyhow.