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WCAG: Best Practice: Banner landmark must not be contained in another landmark
Banner landmark must not be contained in another landmark.
If the banner landmark is not the top-level landmark (and is contained within another landmark), it does not effectively designate the pre-defined header portion of the layout in the design and therefore prevents screen reader users from being able to easily find their way around the layout.
Landmarks are used to designate sections of the overall page design and layout. Headings are used to designate sections within the content.