scrollable-region-focusable
Scrollable regions with no focusable content should be keyboard-focusable
Flags elements whose CSS declares overflow:auto/scroll, contain no focusable descendant, and are not themselves keyboard-focusable, for manual review of whether their content actually overflows and needs keyboard scroll access.
Applies to. Scoped on purpose to a fixed set of likely-to-scroll container tags (div, section, article, aside, main, nav, pre, table, blockquote, ul, ol, textarea) with computed `overflow-x`/`overflow-y` of `auto` or `scroll`, not every element on the page, to keep this deterministic and performant (same style of scope-down as `region`).
Expectation. A region whose CSS declares it may scroll (`auto`/`scroll`) should be reachable by keyboard: either it already contains a focusable descendant (a link, button, form control, or `tabindex`-bearing element a keyboard user could tab into and then use arrow keys to scroll from), or the region itself carries a non-negative `tabindex`.
Examples
<style>.scrollY { overflow-y: auto; height: 100px; }</style><div class="scrollY">Plain long text content with no interactive children at all.</div>
The region can scroll, but it has no `tabindex` of its own and contains no focusable descendant, so a keyboard user can never reach it to scroll it.
<style>.scrollY { overflow-y: auto; height: 100px; }</style><div class="scrollY" tabindex="0">Long content, but the region itself is focusable.</div>
`tabindex="0"` makes the scrollable region itself reachable by keyboard.