role="presentation"表示什么意思？有什么用？

 <li role="presentation"><a role="menuitem" tabindex="-1" href="#">下拉菜单项</a></li>

    <li role="presentation"><a role="menuitem" tabindex="-1" href="#">下拉菜单项</a></li>

    <li role="presentation"><a role="menuitem" tabindex="-1" href="#">下拉菜单项</a></li>

    <li role="presentation" class="divider"></li>

    <li role="presentation"><a role="menuitem" tabindex="-1" href="#">下拉菜单项</a></li>