JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    Lorem ipsum ipsum ipsum.
    <div class="bottom-warning">This site requires an HTML rendering engine!</div>
</body>

CSS

body, html {
    margin: 0;
    padding: 0;
}

.bottom-warning {
    background-color: lightblue;
    box-sizing: border-box;
    left: 0;
    padding:0 20px;
    position: fixed;
    width: 100%;
    bottom:-20px;
    transition:all 1s ease-in-out;
    -webkit-transition:all 1s ease-in-out;
    height:0px;
    overflow:hidden;
}

.up{height:auto;padding:20px;bottom:0px;}

JavaScript

$(function () {
    
    setTimeout(function () {
        $( ".bottom-warning" ).addClass("up");
    }, 500);
});