On a website, which is the correct error message when a user does not provide their username, their password, or both:
No username or password provided.
No username or no password provided.
No username and/or no password provided.
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No, at least not at first glance. The first: "No username or password provided." will usually be read as meaning neither a username nor a password was provided, i.e. both fields are empty.
The second: "No username or no password provided." means that one of the two is missing, but not necessarily both.
The third: "No username and/or no password provided." is not a way you'd typically see the situation phrased, and is sort of a combination of the two, covering both the case where one and neither are entered.