JSFiddle - React, Tailwind, and code Playground

HTML

<div id="lc">&nbsp;</div>
<iframe id="edit"></iframe>

CSS

#lc { background: #eeeeee; margin-bottom: 5px; width: 200px; border: 1px solid #bbbbbb; }
#edit { width: 200px; border: 1px solid #bbbbbb; }

JavaScript

var f = $('iframe#edit')[0].contentWindow.document;

f.designMode = 'on';

$(f).keyup(function() {
   var x = $(this).contents().children('body').children();
   $('#lc').text(x.length); // This will be the line number    
});