HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>ariaMultiSelectable MDN</title>
</head>
<body>
  <div id="listbox1label">Colors</div>
  <div
    role="listbox"
    tabindex="0"
    id="listbox1"
    aria-multiselectable="true"
    aria-labelledby="listbox1label"
    aria-activedescendant="listbox1-1"
  >
    <div role="option" id="listbox1-1" class="selected" aria-selected="true">Green</div>
    <div role="option" id="listbox1-2">Orange</div>
    <div role="option" id="listbox1-3">Red</div>
  </div>
  <pre id="out"></pre>
  <script>
    var el = document.getElementById("listbox1");
    var lines = [el.ariaMultiSelectable];
    el.ariaMultiSelectable = "false";
    lines.push(el.ariaMultiSelectable);
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Element.ariaMultiSelectable — MDN set false (Try It)