H4 Example 2: Creating a logical tab order through links, form controls, and objects
Tabindex values need not be sequential nor must they begin with any particular value. The values do not have to be unique. Elements that have identical tabindex values are navigated in the order they appear in the character stream.
In sections of the content where the tab order follows the content order, it can be less error prone to give all elements the same tabindex value rather than specifying a different number for each element. Then it is easy to rearrange those elements or add new elements and maintain a logical tab order.