JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
    <button type="button" id="clck" onclick="javascript:popup();">Clicky?</button>
</div>


<div id="alertContainer">
    <div id="alertContent">
        <p>
            I fight dragons with tooth picks. Rawrr!
        </p>        
        <span id="close">&times;</span>
    </div>    
</div>

<script>
    var popupBox = document.getElementById('alertContainer');
    var x = document.getElementById('close');
    
    function popup(){    
        popupBox.className = popupBox.className + 'show';
    }   
    
</script>

CSS

#main {
    width: 98vw;
    height: 97vh;
    background-color: #1abc9c;
}

#clck {
    margin-top: 15vh;
    margin-left: 20vw;
    height: 40px;
    width: 90px;
    border-style: none;
    background-color: #2ecc71;
    color: #fff;
}

#clck:hover{
    background-color: #9b59b6;
}

#clck:focus {
    outline: none; 
}

#alertContainer {
    position: absolute;
    display: none;
    width: 350px;
    height: 200px;
    top: 35%;
    left: 30%;
    background-color: #e67e22;
    color: #fff;
    text-align: center;
    line-height: 100px;
}

#alertContainer.show {
    display: block;   
}

#close {
    font-size: 2em;
}

#close:hover {
    color: #9b59b6;
    cursor: pointer;
}

JavaScript

$('#close').click(function() {
    popupBox.className = "";
});