[focusgroup] Should focusgroup not consider positive tabindexes as different from '0'?

[janewman]

A quick search for "positive tabindex" will give a myriad of articles asking developers to not, but the current focusgroup does allow for these positive values to impact behavior

e.g. https://www.scottohara.me/blog/2019/05/25/tabindex.html

I thought reading flow nicely discouraged use of positive tab indexes by not making them useful, https://developer.mozilla.org/en-US/docs/Web/CSS/reading-order#interaction_with_tabindex, I think we should do the same for focusgroup.