Things like alert say:

If the active sandboxing flag set of the active document of the responsible browsing context specified by the incumbent settings object has the sandboxed modals flag set, then abort these steps.

This makes no sense to me. If you have the incumbent settings already, why would you then go through the browsing context to the active document, which may have absolutely nothing to do with the incumbent global's most recent document?

@mikewest it looks like you wrote this bit. What was the intent here?