Are interface requirements functional or non-functional?

Interface requirements are usually considered non-functional requirements because they describe how the system should interact with the user, ensuring usability, ergonomics, accessibility, and aesthetic aspects. However, if the interface describes specific actions or functions that the system must perform (for example, a button must trigger a certain operation), then these are functional requirements. Overall, interface requirements are more often classified as non-functional because they affect the quality of interaction rather than business logic.