If turned on, this rule will make sure that all focusable elements either
have a positive `tabindex` or do not.

focusable elements: a, area, button, input, img, select, textarea

Reasoning: [IITAA, 10.3 and 10.4](http://www.dhs.state.il.us/IITAA/IITAAWebImplementationGuidelines.html)