`aria-level` is listed as supported in [ARIA 1.2](https://www.w3.org/TR/wai-aria-1.2/#treeitem) but not the [latest editor draft](https://w3c.github.io/aria/#treeitem) However, the [description of `aria-level`](https://w3c.github.io/aria/#aria-level) still mentions treeitems as supported. May have been an unintended side-effect of https://github.com/w3c/aria/pull/1484 (previously `aria-level` would be inherited from `listitem`).