[MrJohz]

You almost certainly don't want to assign tab order manually. Theoretically, if your html is out-of-order for some reason, then you can fix it with taborder, but there are so many issues with out-of-order html that it's much better to avoid that in the first place. Even on modern websites, it is almost always easier to lay your html out in the correct order rather than messing around with the order via CSS. (I read an article recently by a web accessibility expert that discussed the issue of flexbox's order parameter and how the working group designing it failed to accessibility into account when adding it to the spec: https://alice.boxhall.au/articles/a-threat-model-for-accessi...)

You mention using it to skip a nav header or something similar, but (1) you have autofocus for that if you need it (and you don't usually need it), and (2) the pattern where you provide a "jump to content" button is typically a better approach, because it allows the user to decide what they want to do when they arrive on your site, rather than you deciding for them. If the "jump to content" button behind visible when focused and is the first element on the screen, then it works perfectly for screen readers and other keyboard users, and you don't need to handle taborder manually.

There are always exceptions to these sorts of rules, and some times tabindex might be necessary, but I've not yet come across a case where the problem couldn't be solved better in a different way.