点击空白处收起菜单实现方法

<div class="menuc">
    <a href="javascript:" class="menu-item" tabindex="0">菜单</a> 
    <div class="sub-menu">
        <a href="https://www.bing.com/">子菜单跳转</a>
        <a href="javascript:;" onclick="onMsg()">子菜单触发事件</a>
    </div>
</div>
<script>
    function onMsg(e){
        alert('触发事件')
    }
</script>

方法1 :focus-within

.sub-menu {
    display: none;
    position: absolute;
}

.menuc:focus-within .sub-menu {
    display: block;
} 

方法2 :focus

.menu-item:focus ~ .sub-menu, .sub-menu:active{
    display: block;
}