About This HTML Issue
The aria-multiselectable attribute only accepts the values true and false, written in lowercase with no spaces around them.
aria-multiselectable tells assistive technologies whether users can select more than one item in a widget. It belongs on elements with the role listbox, grid, tablist, tree or treegrid, and its default value is false. Screen readers use it to announce a list as multi-select, so users know they can pick several options.
The validator rejects every other value. Leading spaces (" true"), a trailing space ("true "), capital letters ("TRUE"), words like "yes", the number "1" and an empty value all trigger this error. Browsers may treat a value they don't recognize as the default, so screen readers announce the widget as single-select even when the script lets users pick several items. Stray spaces often come from a template that prints whitespace or a line break before the value it inserts.
Change the value to exactly true or false. If the widget allows only one selection, you can also drop the attribute, since false is the default. On a div with no role, the attribute has no meaning, and the validator also reports that the div is missing a role attribute.
Invalid example
<div role="listbox" aria-label="Toppings" aria-multiselectable=" true">
<div role="option" aria-selected="true">Cheese</div>
<div role="option" aria-selected="false">Olives</div>
</div>
Valid example
<div role="listbox" aria-label="Toppings" aria-multiselectable="true">
<div role="option" aria-selected="true">Cheese</div>
<div role="option" aria-selected="false">Olives</div>
</div>
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