Keyboard-only testing means navigating an entire page using Tab, Shift+Tab, Enter, Space, and arrow keys alone, checking that every link, button, form field, and custom widget can be reached in a logical order, operated correctly, and shows a clearly visible focus indicator at every step.
This single test surfaces some of the most common and most damaging accessibility failures: focus traps that strand a user inside a modal, custom dropdowns that only respond to a mouse click, and invisible focus outlines that leave a sighted keyboard user with no idea where they currently are on the page.