Set size and position

A screen reader counts what it can see in the accessibility tree. Land on a row and it says “item 3 of 6”, because six rows is what the tree contains. That is a fact about your DOM, not about your data, and the moment the DOM stops holding the whole list the fact becomes a lie. aria-setsize on each item declares the real total, aria-posinset declares that item’s real place in it, and together they let a reader announce “item 247 of 500” from a window that only ever renders six rows.

This is the accessibility half of list virtualization, and it is the half that gets forgotten, because nothing looks broken. Sighted users have a scrollbar whose thumb tells them how much list there is. A reader user without these attributes has no total at all, no way to judge how deep they are, and no way to tell whether the list ended or the renderer simply has not caught up. The same pair fixes paginated lists, where page two starts at item 1 again by default, and infinitely loading feeds, where the total genuinely changes as more arrives.

The mechanics are ordinary and the constraints are worth memorising. Both attributes are integers and both belong on the items, not on the container, and every item in the set needs them (a set where half the rows declare a size gets announced inconsistently). aria-setsize="-1" is the declared way to say the total is unknown, which is the honest value for a feed still loading. Positions are one-based. And the pair only works on roles that are actually members of a set: listitem, option, row, treeitem, menuitem, tab, article inside a feed. On a plain div they mean nothing at all.

One warning about announcing changes. A count is context, not news. If your list grows while somebody is reading it, updating aria-setsize is right and shouting the new total through a polite region is usually wrong, because a total repeated at every arrival becomes noise. Where you do announce something, make sure the announcement is a whole thought rather than a fragment, which is what atomic live region is about.