You'll always need a number to order tabindex. So there's the solution with html only (css is only for the visual on this example but does not affect to functionallity).
You can use css to reach this too but you still need a number and will need an id for each one (with different, ordered numbers on classes) and its more lines than doing it as this.
a{text-decoration:none; list-style:none; display:inline-block; margin:10px; color:#000;}
div{background-color:#DDD; padding:15px;}
<a href="#"><div tabindex="1">lasasdsa</div></a>
<a href="#"><div tabindex="2">ghfghfh</div></a>
<a href="#"><div tabindex="3">dafs5d</div></a>
Run snippet, click on white space of page, and press tab twice, then shift + tab. Works as there are form-controls or links.
Hope it helps, Cheers!