The website archiveofourown.org has a handy feature to directly download the entire story straight into epub format
The epub they provide you even contains syntax for url identifier where you downloaded it from.
From the builtin "edit book" option in calibre
below is the first few lines of the main file of the epub:
*~*~*~*~*~*~*
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<meta content="ie=edge" http-equiv="x-ua-compatible"/>
<title>The Way of Serenity</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<link rel="stylesheet" type="text/css" href="stylesheet.css"/>
<link rel="stylesheet" type="text/css" href="page_styles.css"/>
</head>
<body class="calibre">
<div id="preface" class="calibre1">
<h2 class="toc-heading" id="calibre_toc_2">Preface</h2>
<p class="message">
<b class="calibre2">The Way of Serenity</b><br class="calibre1"/>
Posted originally on the <a href="http://archiveofourown.org/">Archive of Our Own</a> at <a href="https://archiveofourown.org/works/63545731">https://archiveofourown.org/works/63545731</a>.
</p>
*~*~*~*~*~*~*
Would there be any way to use Calibre's conversion process to scrape this url text and use it as the identifer url (or just use it in the comments text)?