[bugsliker]

I'm saying tabindex=0 is naturally sorted wrt other naturally focusable elements. That matches the behavior of the <button> you're trying to emulate. I don't know what tabindex>0 has to do with this.

See this fiddle https://jsfiddle.net/483uqjnp/

(again, I do not condone building your own <button>, just pointing this out)