代码用文档方式给大家，直接插入到你们网站里面就行了，PC端和移动端都适用有点基础的可以改下代码让他更好看！
代码如下：

<div class="web_notice" style="
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.3);
    z-index: 99999;
">
<div style="
    position: fixed;
    top: 50%;
    left: 50%;
    width: 550px;
    background: #FFF;
    transform: translate(-50%, -50%);
    border-radius: 40px;
    padding: 50px 40px;
">
<h3 style="
    font-weight: bold;
    text-align: center;
    font-size: 30px;
">网站通知</h3>
<div style="
    font-size: 16px;
    margin-top: 26px;
    line-height: 30px;
    color: #999;
">公告</div><a style="
    display: block;
    background: #98a3ff;
    color: #FFF;
    text-align: center;
    font-weight: bold;
    font-size: 19px;
    line-height: 60px;
    margin: 0 auto;
    margin-top: 45px;
    border-radius: 32px;
    width: 80%;
" onclick="javascript:document.querySelector('.web_notice').remove()">我知道了</ a>
</div>
</div>