HTML Guide for aria-setsize

Due to an issue in the W3C validator, this is identified as an error but it’s not. This issue has been notified, and we’ll update our validator as soon as there’s a fix.

According to the WAI-ARIA 1.2 spec:

The aria-setsize property defines the number of items in the current set of listitems or treeitems. Not required if all elements in the set are present in the DOM.

Authors MUST set the value of aria-setsize to an integer equal to the number of items in the set. If the total number of items is unknown, authors SHOULD set the value of aria-setsize to -1.