Callout for labelling of input groups

plus a few suggested approaches (in some cases, elided, as the actual
content of the label - being a generic example - is not important)
Use aria-label for pagination prev/next

Cleaner alternative to using ``<span class="sr-only">``
Make main segmented button keyboard-focusable

unless there's a reason why ``tabindex="-1"`` was dropped in there...