This covers keyboard accessibility (every interactive element must be reachable and usable via Tab, Enter, and arrow keys), giving users enough time to read and act on content, avoiding content that flashes in a way that could trigger seizures, and providing ways to navigate, find content, and determine where you are (like page titles, focus order, and skip links).
Operable failures, like a dropdown menu that only opens on mouse hover, are some of the most disabling for keyboard-only and switch-device users, even when the site looks fine visually.