Short for "position in set", the aria-posinset attribute defines the position of the element within the whole set of listitems or treeitems when only a subset of the items are present in the DOM.
If all the items in a list are present in the DOM, the browser can calculate the total and the position of each, making aria-posinset unnecessary. When only a portion of a set is in the DOM, include aria-posinset to provide information about the element's position within the set along with aria-setsize to inform the user how many items are in the full set.
The following example shows a listbox with four element options out of the 118 in the periodic table of chemical elements.
<h2 id="periodic-table">Periodic table of chemical elements</h2>
<ul role="listbox" aria-labelledby="periodic-table">
<li role="option" aria-setsize="118" aria-posinset="1">Hydrogen</li>
<li role="option" aria-setsize="118" aria-posinset="3">Lithium</li>
<li role="option" aria-setsize="118" aria-posinset="11">Sodium</li>
<li role="option" aria-setsize="118" aria-posinset="19">Potassium</li>
</ul>
The value of each aria-posinset is an integer greater than or equal to 1, and less than or equal to the size of the set when that size is known.
Note: When using aria-posinset you must also include value for aria-setsize, which is the size of the entire set. If the size of the entire set is unknown, set aria-setsize="-1".
For menuitem, menuitemcheckbox, or menuitemradio, set the value of aria-posinset with respect to the total number of items in the menu, excluding any separators.
In a feed, each <article> element has aria-posinset set to a value that represents its position in the feed along with the aria-setsize set to either the number of articles that have been loaded or the total number in the feed, depending on which value is most helpful to users.
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