Keep the input in default state
Error inputs should be easy to notice — this supports Nielsen's "visibility of system status" heuristic. However, users should notice them only after they enter any data. Ideally, input validation should happen inline. It means the input should be checked for errors only after users enter information and can afford to focus away from the input.
Pro Tip: Use inline validation to indicate successful entries. It'll remove any second-guessing that users may have to do when filling in a complex form with multiple inputs.
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