纯CSS与JavaScript实现下拉菜单及登录弹窗

<style>
    *{
      margin: 0px;
      padding: 0px;
    }
    .menu{
      width: 1100px;
      height: 30px;
      background-image: url(img/魅力罗兰Music炫图18.jpg);
      margin-left: 200px;
      margin-top: 50px;
    }
    .btn{
      width: 183.3px;
      height: 30px;
      float: left;
      text-align: center;
      line-height: 30px;
      font-size: 14px;
      position: relative;
      overflow: hidden;
      transition: 0.5s;
    }
    .btn:hover{
      cursor: pointer;
      background-color: burlywood;
      color: white;
      max-height: 200px;
      overflow: visible;
    }
    .btn ul{
      list-style: none;
      background-color: #008000;
    }
    
  </style>
  
  <body>
    <div class="menu">
      <div class="btn">罗兰首页</div>
      <div class="btn">歌曲专栏
        <ul>
          <li>流行</li>
          <li>摇滚</li>
          <li>蓝调</li>
          <li>民谣</li>
        </ul>
      </div>
      <div class="btn">音乐人专栏
        <ul>
          <li>内地</li>
          <li>欧美</li>
          <li>日韩</li>
          <li>港台</li>
        </ul>
      </div>
      <div class="btn">乐器专栏
        <ul>
          <li>钢琴</li>
          <li>小提琴</li>
          <li>吉他</li>
          <li>架子鼓</li>
        </ul>
      </div>
      <div class="btn">戏曲专栏
        <ul>
          <li>京剧</li>
          <li>话剧</li>
          <li>豫剧</li>
          <li>黄梅戏</li>
        </ul>
      </div>
      <div class="btn">魅力ROLAND体验区
        <ul>
          <li>新曲</li>
          <li>唱片</li>
          <li>MV</li>
        </ul>
      </div>
    </div>
    
  </body>

<html>

  <head>
    <meta charset="UTF-8">
    <title></title>
    <style>
      * {
        margin: 0px;
        padding: 0px;
      }
      
      .login {
        width: 100px;
        height: 30px;
        font-size: 25px;
        line-height: 30px;
        border: 1px solid black;
        text-align: center;
        background-color: darkcyan;
        color: white;
      }
      
      .login:hover {
        cursor: pointer;
        background-color: greenyellow;
      }
      
      .mask {
        width: 100%;
        background-color: black;
        opacity: 0.5;
        position: absolute;
        top: 0px;
        left: 0px;
        z-index: 90;
      }
      
      .log-div {
        width: 500px;
        height: 300px;
        background-color: white;
        position: fixed;
        z-index: 99;
      }
    </style>
  </head>

  <body>
    <div class="mask" hidden></div>
    <div class="log-div" hidden="hidden"></div>
    <div class="login">登 录</div>
    <p>123</p><p>123</p><p>123</p><p>123</p><p>123</p><p>123</p><p>123</p>
    <p>123</p><p>123</p><p>123</p><p>123</p><p>123</p><p>123</p><p>123</p>
    <p>123</p><p>123</p><p>123</p><p>123</p><p>123</p><p>123</p><p>123</p>
    <p>123</p><p>123</p><p>123</p><p>123</p><p>123</p><p>123</p>


  </body>

</html>

<script>
  var mask = document.getElementsByClassName("mask")[0];
  var login = document.getElementsByClassName("log-div")[0];
  var btn = document.getElementsByClassName("login")[0];

  var clientWidth = document.body.clientWidth;
  var clientHeight = document.documentElement.clientHeight;

  login.style.left = clientWidth / 2 - 250 + "px";
  login.style.top = clientHeight / 2 - 150 + "px";
  
  mask.style.height = document.body.clientHeight + "px";

  window.onresize = function() {
    var clientWidth = document.body.clientWidth;
    var clientHeight = document.documentElement.clientHeight;

    login.style.left = clientWidth / 2 - 250 + "px";
    login.style.top = clientHeight / 2 - 150 + "px";
  }
  
  btn.onclick = function(){
    mask.removeAttribute("hidden");
    login.removeAttribute("hidden");
  }
  
  mask.onclick = function(){
    mask.setAttribute("hidden","");
    login.setAttribute("hidden","hidden");
  }
</script>