HTML

 <div class="mi">
        <input type="search" placeholder="小米笔记本">
        <ul class="result-list">
            <li><a href="#">全部商品</a></li>
            <li><a href="#">小米11</a></li>
            <li><a href="#">小米10S</a></li>
            <li><a href="#">小米笔记本</a></li>
            <li><a href="#">小米手机</a></li>
            <li><a href="#">黑鲨4</a></li>
            <li><a href="#">空调</a></li>
        </ul>
    </div>

CSS

* {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        ul {
            list-style: none;
        }

        .mi {
            position: relative;
            width: 223px;
            margin: 100px auto;
        }

        .mi input { 
            width: 223px;
            height: 48px;
            padding: 0 10px;
            font-size: 14px;
            line-height: 48px;
            border: 1px solid #e0e0e0;
            outline: none;
            transition: all .2s;
        }

        .mi .search {
            border: 1px solid #ff6700;
            transition: all .2s;
        }

        .result-list {
            display: none;
            position: absolute;
            left: 0;
            top: 48px;
            width: 223px;
            border: 1px solid #ff6700;
            border-top: 0;
            background: #fff;
            transition: all .2s;
        }

        .result-list a {
            display: block;
            padding: 6px 15px;
            font-size: 12px;
            color: #424242;
            text-decoration: none;
        }

        .result-list a:hover {
            background-color: #eee;
        }

JS

 //属性选择器
        const search = document.querySelector('[type=search]')
        //下拉内容
        const result = document.querySelector('.result-list')
        //给搜索框绑定聚焦事件
        search.addEventListener('focus',function(){
            result.style.display = 'block'
            search.classList.add('search')
        })
        //搜索框失焦事件
        search.addEventListener('blur',function(){
      
            result.style.display = 'none'
      
            search.classList.remove('search')
        })