If an element can be focused using the keyboard, then it should be interactive; that is, the user should be able to do something to it and produce a change of some kind (for example, activating a link or changing an option).
Note: One important exception to this rule is if the element has role="document" applied to it, inside an interactive context (such as role="application"). In such a case, focusing the nested document is the only way of returning assistive technology to a non-interactive state (often called "browse mode").
Most interactive elements are focusable by default; you can make an element focusable by adding a tabindex=0 attribute value to it. However, you should only add tabindex if you have also made the element interactive, for example, by defining appropriate event handlers keyboard events.
tabindex attribute greater than zeroThe tabindex attribute indicates that an element is focusable using the keyboard. A value of zero indicates that the element is part of the default focus order, which is based on the ordering of elements in the HTML document. A positive value puts the element ahead of those in the default ordering; elements with positive values are focused in the order of their tabindex values (1, then 2, then 3, etc.).
This creates confusion for keyboard-only users when the focus order differs from the logical order of the page. A better strategy is to structure the HTML document so that focusable elements are in a logical order, without the need to re-order them with positive tabindex values.
If an element can be clicked with a pointing device, such as a mouse, then it should also be focusable using the keyboard, and the user should be able to do something by interacting with it.
An element is clickable if it has an onclick event handler defined. You can make it focusable by adding a tabindex=0 attribute value to it. You can make it operable with the keyboard by defining an onkeydown event handler; in most cases, the action taken by event handler should be the same for both types of events.
If the user can interact with an element using touch or a pointing device, then the element should also support interacting using the keyboard. That is, if you have defined event handlers for touch or click events, you should also define them for keyboard events. The keyboard event handlers should enable the effectively the same interaction as the touch or click handlers.
If the user can interact with an element (for example, using touch or a pointing device), then it should be focusable using the keyboard. You can make it focusable by adding a tabindex=0 attribute value to it. That will add the element to the list of elements that can be focused by pressing the Tab key, in the sequence of such elements as defined in the HTML document.
Any element that can receive keyboard focus should have visible styling that indicates when the element is focused. You can do this with the :focus CSS pseudo-class.
Standard focusable elements such as links and input fields are given special styling by the browser by default, so you might not need to specify focus styling for such elements, unless you want the focus styling to be more distinctive.
If you create your own focusable components, be sure that you also define focus styling for them.