aria-setsize and aria-posinsetaria-setsize defines the total number of items in a set. aria-posinset defines the position of the current item within that set. They are almost always used together, and together they let assistive technologies announce something like "Reports, 1 of 3" when a user navigates to an item.
These attributes are most commonly needed when the full set of items is not all present in the DOM at once — for example, in a virtualised list or a treegrid where some rows are collapsed. Without them, a screen reader can only report what it can see, which may be fewer items than actually exist in the logical structure.
The example below is a flat, fully visible treegrid representing a company file structure, going down to four levels of depth. Because all rows are present in the DOM, the attributes are technically optional here — but the example is designed to let you inspect and test them directly, including how the count resets for each sibling group at each level.
This treegrid has three top-level departments, each containing folders and files at varying depths. The deepest items sit at level 4. All rows are visible at once, making it straightforward to inspect the aria-setsize and aria-posinset values with a bookmarklet.
The diagram below shows how aria-setsize and aria-posinset are scoped to sibling groups, not to the treegrid as a whole. Each group restarts its own count. R1 is the header row counted by aria-rowcount.
R1: aria-rowcount: 22 (1 header row + 21 data rows)
R2: Finance (Level 1, 1 of 3)
R3: Budget (Level 2, 1 of 2)
R4: 2025.xlsx (Level 3, 1 of 2)
R5: 2026.xlsx (Level 3, 2 of 2)
R6: Reports (Level 2, 2 of 2)
R7: Q3.pdf (Level 3, 1 of 1)
R8: Engineering (Level 1, 2 of 3)
R9: Frontend (Level 2, 1 of 2)
R10: src (Level 3, 1 of 2)
R11: Button.jsx (Level 4, 1 of 2)
R12: Modal.jsx (Level 4, 2 of 2)
R13: tests (Level 3, 2 of 2)
R14: Backend (Level 2, 2 of 2)
R15: api.md (Level 3, 1 of 1)
R16: Marketing (Level 1, 3 of 3)
R17: Campaigns (Level 2, 1 of 2)
R18: Spring2026 (Level 3, 1 of 1)
R19: brief.docx (Level 4, 1 of 1)
R20: Assets (Level 2, 2 of 2)
R21: logo.svg (Level 3, 1 of 2)
R22: brand.pdf (Level 3, 2 of 2)Expected result: navigating through rows, a screen reader should announce each item's position within its sibling group (for example "2 of 2") and its level. The counts reset for each group — the two level-3 items inside Budget are "1 of 2" and "2 of 2", completely independent of the level-3 items inside Frontend or Marketing.
Note for testing: confirm with your standard browser and screen reader combinations that level and position announcements are consistent across different screen reader and browser pairings.