Sets the tabindex for the object.
Tab selection order is determined by the value of tabIndex as follows:
First all elements with TABINDEX > 0 are selected in increasing tabIndex
order, or in source order for duplicate TABINDEX values
Next all elements with tabIndex = 0, or without tabIndex set, in source order
if more than one.
Elements with tabIndex = -1 are omitted from tab selection.
INPUT, OBJECT, SELECT, TEXTAREA