2.1.1 Keyboard
Official description of the success criterion
All functionality of the content is operable through a keyboard interface without requiring specific timings for individual keystrokes, except where the underlying function requires input that depends on the path of the user's movement and not just the endpoints.
What to do?
Ensure all functionality is operable through a keyboard interface.
Why is it important?
Supports users who cannot use a mouse.
Common pitfalls
Sometimes developers use <div> elements instead of <button> elements. Divs don't have keyboard support by default which means that unless it is specifically created for them, the element cannot be accessed at all with a keyboard.
How to test for it?
Check that it is possible to access all content with only a keyboard.
Some automated tools can find issues with this and can also help check the keyboard tab order. But I usually prefer to do the testing manually because it also gives insight into screen reader accessibility.