The way axe-core checks whether tabindex is valid is by calling parseInt(). That's different from how HTMLhttps://html.spec.whatwg.org/#rules-for-parsing-integers says to do it. Basically, axe should use this to match, and then parseInt the captured group: /^\s*([-+]?\d+)/

The issue is here: lib/commons/dom/is-focusable.js
The tests for it need to be here: test/commons/dom/is-focusable.js