The content is intended for backup in case the browser cannot display the object for any reason. Think of it as an attribute altfor elements <img>. Automatic verification cannot verify that you are investing something reasonable, only that you have invested something.
It is your responsibility to ensure that this content is reasonable. It should provide the best equivalent in text (or other HTML) that you can provide that conveys the same information that would be conveyed by the object if it were to be displayed.
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