JSFiddle - React, Tailwind, and code Playground

HTML

<div id="popup">
        <a href="#" onclick="document.getElementById('popup').style.display='none';return false;" id="close_popup"></a>
        <p>This is a pop-up.</p>
</div>

CSS

#popup{
    width:240px;
    background:orange;
    position:absolute;
    top:5px;
    left:5px;
}
#close_popup{
display:block;
width:24px;
height:24px;
background:url('http://cdn3.iconfinder.com/data/icons/gnomeicontheme/24x24/actions/window-close.png');
float:right;
}