网页导航栏鼠标移上去自动弹出下拉菜单是怎么做的？

最简单的是hover
js的话用mouseenter事件

<meta charset="UTF-8" />
<div class="menubar">
    <div class="menuitem">
        <div>菜单1</div>
        <div class="submenu">
            <div>子菜单1</div>
            <div>子菜单2</div>
        </div>
    </div>
    <div class="menuitem">
        <div>菜单2</div>
        <div class="submenu">
            <div>子菜单1</div>
            <div>子菜单2</div>
            <div>子菜单3</div>
        </div>
    </div>
    <div class="menuend"></div>
</div>
<p>
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</p>
<style type="text/css">
    .menubar
        {line-height: 24px;}
    .menubar .menuend
        {clear: both;}
    .menuitem
        {background: #fff; border: 1px solid #c00; position: relative; float: left; margin-right: 1em;}
    .menuitem .submenu
        {background: #ccc; border: 1px solid #00c; position: absolute; top: 25px; left: -1px; width: 5em;}
    /** 下面的控制显示和隐藏 **/
    .menuitem .submenu
        {display: none;}
    .menuitem:hover .submenu
        {display: block;}
</style>