Aside from Bootstrap's alerts page I am unable to find any material guidelines for alerts that we can use in the development of a notification bar.
Are there any good guidelines out there based on user behaviour?
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Under the Snackbar section, it recommends which component to use based on the amount of interruption or attention that's needed from the user:
I don't know your use case, or your definition of 'Alert', but you have two other options:
Alert dialogs interrupt users with urgent information, details, or actions.
A banner displays an important, succinct message, and provides actions for users to address (or dismiss the banner). It requires a user action to be dismissed.