The tabindex attribute allows authors to indicate that an element is supposed to be focusable, whether it is supposed to be reachable using sequential focus navigation and, optionally, to suggest where in the sequential focus navigation order the element appears.
|negative||no sequential focus|
|omitted||user agent decides focus|
|positive||sequential focus with order by number|
|zero||sequential focus with order by placement|
Web Hypertext Application Technology Working Group (WHATWG)
World Wide Web Consortium (W3C)