tabindex=-1在这里有什么作用

 <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">

    <li role="presentation" class="dropdown-header">第一部分菜单头部</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"><a role="menuitem" tabindex="-1" href="#">下拉菜单项</a></li>