code: function(val) {
      if (/^(?:auto|inherit)$/.test(val)) {
        return val;
      }
      var zIndex;
      if (typeof val === "number" && !_isNaN(val)) {
        zIndex = val;
      } else if (typeof val === "string") {
        zIndex = _getSafeZIndex(_parseInt(val, 10));
      }
      return typeof zIndex === "number" ? zIndex : "auto";
    }
docs: Get a safe value for `zIndex`
  
  @returns an integer, or "auto"
  @private