Code:

<div id="myNav" class="overlay">
    <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>
    <div class="overlay-content">
        <a href="#">About</a>
        <a href="#">Services</a>
        <a href="#">Clients</a>
        <a href="#">Contact</a>
    </div>
</div>

<h2>全屏幕导航 - 从上到下动画</h2>
<p>点击以下菜单图标打开导航菜单</p>

<span style="font-size:30px;cursor:pointer" onclick="openNav()">&#9776; 打开</span>
<style>
    body {
        margin: 0;
        font-family: 'Lato', sans-serif;
    }
    
    .overlay {
        height: 0%;
        width: 100%;
        position: fixed;
        z-index: 1;
        top: 0;
        left: 0;
        background-color: rgb(0,0,0);
        background-color: rgba(0,0,0, 0.9);
        overflow-y: hidden;
        transition: 0.5s;
    }
    
    .overlay-content {
        position: relative;
        top: 25%;
        width: 100%;
        text-align: center;
        margin-top: 30px;
    }
    
    .overlay a {
        padding: 8px;
        text-decoration: none;
        font-size: 36px;
        color: #818181;
        display: block;
        transition: 0.3s;
    }
    
    .overlay a:hover, .overlay a:focus {
        color: #f1f1f1;
    }
    
    .overlay .closebtn {
        position: absolute;
        top: 20px;
        right: 45px;
        font-size: 60px;
    }
    
    @media screen and (max-height: 450px) {
      .overlay {overflow-y: auto;}
      .overlay a {font-size: 20px}
      .overlay .closebtn {
        font-size: 40px;
        top: 15px;
        right: 35px;
      }
    }
</style>

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