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WCAG: Best Practice: All page content must be contained by landmarks
It is best practice to contain all content excepting skip links, within distinct regions such as the header, nav, main, and footer.
Navigating a web page is far simpler for screen reader users if the content splits between multiple high-level sections. Content outside of sections is difficult to find, and the content's purpose may be unclear.
Historically, HTML lacked some key semantic markers such as the ability to designate sections of the page as the header, navigation, main content, and footer. Using both HTML5 elements and ARIA landmarks in the same element is considered a best practice, but the future favors using native HTML5 element regions as browser support increases.