Html css下拉菜单阻止父菜单

这是我第一次尝试构建一个合适的水平菜单，但我遇到了一点问题。当我将鼠标悬停在子菜单上时，它的子菜单会覆盖它。在Chrome、Firefox和IE上测试，都有相同的问题。我整个星期都在寻找一个解决方案，所以任何帮助都将不胜感激。这是菜单

    <li><a>Courses</a>
 <ul>
  <li><a>RII31813</a>
   <ul>
    <li><a>Student Information</a></li>
    <li><a>BSBWHS301A Maintain workplace saftey</a></li>
    <li><a>RIIBEF301D Run on-site operations</a></li>
    <li><a>RIICOM201D Communicate in the workplace</a></li>
    <li><a>RIINHB301D Set up and prepare for drilling operations</a></li>
    <li><a>RIIRIS301D Apply risk management processes</a></li>
    <li><a>RIISAM209D Carry out operational maintenance</a></li>
    <li><a>MSAPMSUP172A Identify and minimise environmental hazards</a></li>
    <li><a>AEDEL301A Provide work skill instruction</a></li>
    <li><a>RIINHB307D Conduct conventional core drilling</a></li>
    <li><a>RIINHB308D Conduct wireline core drilling</a></li>
    <li><a>RIINHB324D Carry out grouting or cementing operations</a></li>
    <li><a>RIINHB208D Assist diamond core drilling</a></li>
    <li><a>RIIQUA201D Maintain and monitor site quality standards</a></li>
    <li><a>RIIVEH305D Operate and maintain a four wheel drive vehicle</a></li>
   </ul>
  </li>
 </ul>
</li>

---

感谢您的帮助。

注意：我不知道原始的HTML代码，我认为这可能是正确的HTML并继续使用

HTML:

  <ul id="menu">
    <li><a>Courses</a>
        <ul>
        <li><a>RII31813</a>
        <ul>
            <li><a>Student Information</a></li>
            <li><a>BSBWHS301A Maintain workplace saftey</a></li>
            <li><a>RIIBEF301D Run on-site operations</a></li>
            <li><a>RIICOM201D Communicate in the workplace</a></li>
            <li><a>RIINHB301D Set up and prepare for drilling operations</a></li>
            <li><a>RIIRIS301D Apply risk management processes</a></li>
            <li><a>RIISAM209D Carry out operational maintenance</a></li>
            <li><a>MSAPMSUP172A Identify and minimise environmental hazards</a></li>
            <li><a>AEDEL301A Provide work skill instruction</a></li>
            <li><a>RIINHB307D Conduct conventional core drilling</a></li>
            <li><a>RIINHB308D Conduct wireline core drilling</a></li>
            <li><a>RIINHB324D Carry out grouting or cementing operations</a></li>
            <li><a>RIINHB208D Assist diamond core drilling</a></li>
            <li><a>RIIQUA201D Maintain and monitor site quality standards</a></li>
            <li><a>RIIVEH305D Operate and maintain a four wheel drive vehicle</a></li>
        </ul>
    </li>
 </ul>

---

这里的工作很好：

所有CSS都使用

#menu

，但是没有ID为

menu

的元素，哈哈。感谢您尝试将问题本地化，但是我们需要更多的HTML来帮助您非常轻松地创建一个演示。请在下面创建一个演示我的答案是否有效？谢谢您，我以前也尝试过，但经过一点修补和一些额外的ID，它成功地工作了，谢谢！

  <ul id="menu">
    <li><a>Courses</a>
        <ul>
        <li><a>RII31813</a>
        <ul>
            <li><a>Student Information</a></li>
            <li><a>BSBWHS301A Maintain workplace saftey</a></li>
            <li><a>RIIBEF301D Run on-site operations</a></li>
            <li><a>RIICOM201D Communicate in the workplace</a></li>
            <li><a>RIINHB301D Set up and prepare for drilling operations</a></li>
            <li><a>RIIRIS301D Apply risk management processes</a></li>
            <li><a>RIISAM209D Carry out operational maintenance</a></li>
            <li><a>MSAPMSUP172A Identify and minimise environmental hazards</a></li>
            <li><a>AEDEL301A Provide work skill instruction</a></li>
            <li><a>RIINHB307D Conduct conventional core drilling</a></li>
            <li><a>RIINHB308D Conduct wireline core drilling</a></li>
            <li><a>RIINHB324D Carry out grouting or cementing operations</a></li>
            <li><a>RIINHB208D Assist diamond core drilling</a></li>
            <li><a>RIIQUA201D Maintain and monitor site quality standards</a></li>
            <li><a>RIIVEH305D Operate and maintain a four wheel drive vehicle</a></li>
        </ul>
    </li>
 </ul>

#menu ul ul {
    top: 0;
    left: 120px;
    float:right;
    margin: 0 0 0 20px;
    _margin: 0; /*IE6 only*/
    box-shadow: -1px 0 0 rgba(255,255,255,.3);      
}