tabindex assignments are handled the following way (for elements that support the tabindex attribute):
- Positive numbers (1,2,3…32767) are handled in tab order.
- 0 is handled in source order (the order it appears in the DOM)
- -1 is ignored during tabbing but is focusable.
This information is taken from : http://www.w3.org/TR/html401/interact/forms.html#adef-tabindex