How to tell if there is an error message page in firefox

https://support.mozilla.org/z.

A competitor"s site is embedded in iframe. I hope to do something when the "your connection is not secure" page" appears.
how can I tell if there is an "error message page"?


js should be able to listen for events corresponding to iframe, or will there be a status code?
anyway, the general idea is to listen for the event and then execute the processing.

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