Specifies or returns the tabbing order for keyboard navigation using the TAB key.
The default tabbing order matches the source order in the markup, with tabIndex you can change this order. The browser will tab to the elements in ascending order.
The another effect of the tabIndex property is that if it is specififed, the element can be active (can have focus). Most of the HTML elements cannot be active by default (div, span, ...), but if the tabIndex property is set on them, they can be active.