JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://netdna.bootstrapcdn.com/bootstrap/3.0.1/css/bootstrap.min.css">
<script src="http://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.10.2/jquery-ui.min.js"></script>
<script src="//netdna.bootstrapcdn.com/bootstrap/3.0.1/js/bootstrap.min.js"></script>
<textarea id="userinput">
    <h1>test</h1><b>bbb</b>
</textarea>
<div id="output"></div>

CSS

h {
    color:red;
}

JavaScript

var message = $('<div/>', {
    html: document.getElementById("userinput").value
}).find(':header').replaceWith(function () {
    return '<h>' + this.innerHTML + '</h>';
}).end().html();
$('#output').html(message);