To explain why I had to change this to `"auto"` instead of `0`:
 - This assertion previously passed because `window.getComputedStyle(el, null).getPropertyValue("zIndex")` returns an empty value.
 - NodeUnit's `test.equal` does a type-coercive equality check (`==`) rather than a strict equality check (`===`), so `"" == 0` evaluated to `true`. Switching to `test.strictEqual` (`===`) would've caught this earlier.
 - Now that I've fixed the computed style lookup, the value returned is `"auto"` instead of an empty value, which will _not_ equal `0` even under type-coercive equality checks. :wink: