There are many different ways to use the web besides a mouse and a pair of eyes. Users navigate for example with a keyboard only or with their voice.
All the functionality, including menus, links and forms should work using a keyboard only. This is essential for all assistive technology to work properly. The only way to test this, at the moment, is manually. The best time to test this is during development.
How to keyboard test:
Tab through your pages, links and forms to do the following tests:
Confirm that all links can be reached and activated via keyboard, including any in dropdown submenus.
Confirm that all links get a visible focus indicator (e.g., a border highlight).