<style>
        * {
            margin: 0;
            padding: 0;
        }
        .center-wrap {
            position: relative;
            box-sizing: border-box;
            font-size: 14px;
            width: 1152px;
            margin: 0 auto;
            margin-top: 30px;
        }

        .log {
            float: left;
            font-size: 24px;
        }

        .slide-nav {
            float: right;
            position: relative;
        }

        .slide-nav ul {
            width: 360px;
            overflow: hidden;
        }

        .slide-nav li {
            float: left;
            width: 50px;
            height: 30px;
            list-style: none;
            margin-right: 10px;
            cursor: pointer;
        }

        .slide-nav ul li:hover {
            color: #f01400;
        }

        .slide-nav li:last-child {
            margin-right: 0;
        }

        .slide-nav li a {
            text-decoration: none;
            color: black;
        }

        .btn {
            position: absolute;
            right: 0;
            background-color: black;
            color: wheat;
        }

        .em {
            width: 30px;
            height: 4px;
            background-color: #f01400;
            position: absolute;
            top: 20px;
            left: 0;
            transition: left 0.5s ease 0s;
        }
    </style>
     <section class="center-wrap">
        <div class="log">慕课手机</div>
        <nav class="slide-nav">
            <ul id="list">
                <li data-n="0">
                    <a href="#">首页</a>
                </li>
                <li data-n="1">
                    <a href="#">外观</a>
                </li>
                <li data-n="2">
                    <a href="#">配置</a>
                </li>
                <li data-n="3">
                    <a href="#">型号</a>
                </li>
                <li data-n="4">
                    <a href="#">说明</a>
                </li>
            </ul>
            <div class="em" id="em"></div>
        </nav>
        <button class="btn">立即购买</button>

    </section>
    <script>
        //得到元素
        var em = document.getElementById('em')
        var list = document.getElementById('list')

        //绑定事件监听,用事件委托
        list.onmouseover = function (e) {
            if (e.target.tagName.toLowerCase() == 'li')
                t = e.target.getAttribute('data-n')
            console.log(t)
            em.style.display = 'inline-block'
            em.style.left = 60 * t + 'px'
        }
    </script>
    问题描述：1，我给li设置了：hover属性，为什么鼠标悬停上去的时候li标签的颜色没有
    改变，2，为什么鼠标触碰到li标签的margin区域，添加的监听才会起效果？