Note: On macOS specifically, you may need to enable certain settings for keyboard navigation to work as expected. Go to your Mac Settings, click "Keyboard", click "Shortcuts" and check the "Use keyboard navigation to move focus between controls"
Starting at the top of the page you are evaluating, continuously hit the TAB key and ensure that you can focus on every element on the page that is interactive. This means, all buttons, all links, all form controls, and any other elements that the developer assigned a tabindex value of 0 or higher to.
Some elements may not be visible when you focus on them. Any of those cases need to be marked as part of 2.4.7 Focus Visible. Sometimes you can interact with certain elements even though you are not provided with a visual indication. This guideline is specifically looking for the ability of a keyboard to interact with all interactive elements.
Interacting with elements using a keyboard is dependent on what type of element it is. Below is a list of patterns and how a keyboard is expected to interact with them.
Link: Use the RETURN/ENTER key.
Button: Use either the RETURN/ENTER key or the SPACEBAR. (Both must work to be compliant.)
Text Input: Type once the input has focus.
Radio Button: Use arrow keys to switch between options. (You should NOT use TAB to switch between radio buttons in the same grouping.)
Checkbox: Use RETURN/ENTER or SPACEBAR to tick/untick checkboxes. Checkboxes as part of a group are navigated to using arrow keys.
Select Box: Use RETURN/ENTER or SPACEBAR to open the select box, then use arrow keys to move up and down.
Combobox: Use RETURN/ENTER or SPACEBAR to open the combobox. The text box can be typed in normally, and arrow keys can be used to scroll through the options.
Tablist: Move keyboard focus onto the grouping of tabs. Use Arrow Keys to select the tab you wish to open. Use RETURN/ENTER or SPACEBAR to select the tab. Use TAB to immediately move into the content. (If you have to use the TAB key to navigate through all of the tabs, then it is a failure.)
Accordion: Use RETURN/ENTER or SPACEBAR to expand or collapse the accordion.
Carousel: Use the TAB key to navigate between all of the controls. Use RETURN/ENTER or SPACEBAR to interact with the controls. The use of arrow keys to switch between slides is optional, but if the slide changes, the focus must shift to the active slide instead of being locked to the now-hidden slide.
Navigation: Depending on the implementation, use the TAB key and/or the Arrow keys to select the navigation item, then use the RETURN/ENTER key to click the link. If the navigation offers drop-down functionality, either a button should be available to use RETURN/ENTER or SPACEBAR to open the dropdown, or arrow keys may be used to enter the dropdown. Use the ESC key to return to the parent navigation item, or use the TAB key to navigate out of the dropdown.
Modal: The trigger for a modal should be a button, and therefore triggered using either RETURN/ENTER or SPACEBAR. Within the modal, use TAB to switch between elements, or ESC to immediately close the modal. Modals may also offer an explicit close button which may also be interacted with using RETURN/ENTER or SPACEBAR. If you can use TAB to escape the modal without explicitly closing it, mark a failure under 2.4.3 Focus Order
