tabindex="0" tells browser that an element is focusable. Sometimes that's helpful for accessibility reasons.
At a glance, i see no reason not to allow it, as long as we only allow the value 0 [For reference, positive integers are order of element, we probably don't want users messing with that. -1 means don't allow focusing, which i guess we probably don't want users to do either].
Further read:
https://webaim.org/techniques/keyboard/tabindex#zero