JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://code.jquery.com/jquery.js"></script>
<script src="https://code.jquery.com/jquery.js"></script>
<a href="http://www.zoomtek.dk">ZoomTek Kurser</a><br />
<div id="redbox" class="redbox">Postkasse</div>

CSS

div#redbox{
  position:absolute;
  background-color:red;
  left:100px;
	top:100px;
}

div.redbox{
	width:100px;
	height:100px;
}

div.redboxon{
	color:white;
  width:200px;
	height:200px;
}

JavaScript

$(function(){
  $( "a" ).click(function(event){
  	event.preventDefault();
		$( "#redbox" ).toggleClass("redboxon");
  });
});