二级导航

<!DOCTYPE html>
<html >
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
    <title>erjidaohang</title>
    <link rel="stylesheet" href="erjidaohang.css">
    <script>
    window.onload=function () {
        var oUl=document.getElementById('ul1');
        var aLi=oUl.children;/*请注意子节点只算第一层的，孙子节点不在子节点的范畴内。*/
        for (var i=0;i<aLi.length;i++){
            aLi[i].onmouseover=function () {
                var oSubUl=this.getElementsByTagName('ul')[0];
                if (oSubUl){
                    oSubUl.style.display="block";
                }

            };
            /*在鼠标指针移出指定的对象时执行Javascript代码:*/
            aLi[i].onmouseout=function () {
                var oSubul=this.getElementsByTagName('ul')[0];
                if (oSubul){
                    oSubul.style.display="none";
                }
            };


        }


    };
    </script>




</head>
<body>
    <div class="box">
        <ul class="nav" id="ul1">
            <li><a href="#">page1</a></li>
            <li>
                <a href="#">page2</a>
                <ul>
                    <li><a href="#">item2</a></li>
                    <li><a href="#">item2</a></li>
                    <li><a href="#">item2</a></li>
                    <li><a href="#">item2</a></li>
                </ul>
            </li>
            <li><a href="#">page3</a>
                <ul>
                    <li><a href="#">item3</a></li>
                    <li><a href="#">item3</a></li>
                    <li><a href="#">item3</a></li>
                    <li><a href="#">item3</a></li>
                </ul>
            </li>
            <li><a href="#">page4</a></li>
        </ul>
    </div>
</body>


CSS:

*{
    margin: 0px;
    padding: 0px;
    list-style: none;
    text-decoration: none;
}



.box{
    width: 405px;
    margin: 50px auto;
}
.box .nav{
    margin-left: -1px;
}

.nav:after{
    display: table;
    content: "";
    clear: both; /*清除左右浮动的影响*/
}
.nav>li{
    float: left;
    width: 100px;
    margin-left: 1px; /*排除边界的影响*/
}

/*ransition-property是用来指定当元素其中一个属性改变时执行transition效果
，其中none没有属性改变；all所有属性都改变，它也是默认值；
indent元素属性名，其对应的类型如下：*/
.nav a{
    display: block;
    width: 100%;
    height: 100%;
    text-align: center;
    border: 1px solid #000000;
    color: #000000;
    transition: .1s all ease;
}
.nav a:hover{
    background-color: #eee;
    color: #fff;
}
.nav li{
    height: 30px ;
    line-height: 30px;
}

.nav ul{
    display:none ;
    margin-top: -2px;
}
.nav ul li{
    margin-top: 1px;
}
.active ul{
    display: block;
}

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