Definition
ARIA landmarks are role attributes (or implicit roles from <header>/<main>/<nav>/<footer>) that mark major page regions. They let assistive tech jump between regions and let AI crawlers identify the 'main content' vs chrome. Every page needs exactly one main landmark; missing it is a hard fail in Lighthouse a11y audits and a soft fail in AEO content extraction.
<main>The article text</main> AND a <nav>nav links</nav> AND a <footer>footer chrome</footer>. Engines extract from <main>, ignore footer chrome.
How to optimize
Audit: every page should have one <main>, one <header role='banner'>, one <footer role='contentinfo'>, one or more <nav>. Don't use multiple <main>s.