https://wicg.github.io/import-maps/#document-acquiring-import-maps

An import map is accepted if and only if it is added (i.e., its corresponding script element is added) before the first module load is started

But it doesn't mention onerror must be called, or define what is 'accepted'

The error event is defined later in register an import map

1. If element’s the script’s result is null, then fire an event named error at element, and return.

But the registration is done in almost end of Prepare a script

Prepare a script (2) should have mentioned the error behavior when an import map is added after the 1st module load.