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-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
- "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
-<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head>
-<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
-<meta name="description" content="Explains various methods for allowing IDs in documents safely in HTML Purifier." />
-<link rel="stylesheet" type="text/css" href="./style.css" />
-
-<title>IDs - HTML Purifier</title>
-
-</head><body>
-
-<h1 class="subtitled">IDs</h1>
-<div class="subtitle">What they are, why you should(n't) wear them, and how to deal with it</div>
-
-<div id="filing">Filed under End-User</div>
-<div id="index">Return to the <a href="index.html">index</a>.</div>
-<div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div>
-
-<p>Prior to HTML Purifier 1.2.0, this library blithely accepted user input that
-looked like this:</p>
-
-<pre>&lt;a id=&quot;fragment&quot;&gt;Anchor&lt;/a&gt;</pre>
-
-<p>...presenting an attractive vector for those that would destroy standards
-compliance: simply set the ID to one that is already used elsewhere in the
-document and voila: validation breaks. There was a half-hearted attempt to
-prevent this by allowing users to blacklist IDs, but I suspect that no one
-really bothered, and thus, with the release of 1.2.0, IDs are now <em>removed</em>
-by default.</p>
-
-<p>IDs, however, are quite useful functionality to have, so if users start
-complaining about broken anchors you'll probably want to turn them back on
-with %Attr.EnableID. But before you go mucking around with the config
-object, it's probably worth to take some precautions to keep your page
-validating. Why?</p>
-
-<ol>
- <li>Standards-compliant pages are good</li>
- <li>Duplicated IDs interfere with anchors. If there are two id="foobar"s in a
- document, which spot does a browser presented with the fragment #foobar go
- to? Most browsers opt for the first appearing ID, making it impossible
- to references the second section. Similarly, duplicated IDs can hijack
- client-side scripting that relies on the IDs of elements.</li>
-</ol>
-
-<p>You have (currently) four ways of dealing with the problem.</p>
-
-
-
-<h2 class="subtitled">Blacklisting IDs</h2>
-<div class="subsubtitle">Good for pages with single content source and stable templates</div>
-
-<p>Keeping in terms with the
-<acronym title="Keep It Simple, Stupid">KISS</acronym> principle, let us
-deal with the most obvious solution: preventing users from using any IDs that
-appear elsewhere on the document. The method is simple:</p>
-
-<pre>$config-&gt;set('Attr.EnableID', true);
-$config-&gt;set('Attr.IDBlacklist' array(
- 'list', 'of', 'attribute', 'values', 'that', 'are', 'forbidden'
-));</pre>
-
-<p>That being said, there are some notable drawbacks. First of all, you have to
-know precisely which IDs are being used by the HTML surrounding the user code.
-This is easier said than done: quite often the page designer and the system
-coder work separately, so the designer has to constantly be talking with the
-coder whenever he decides to add a new anchor. Miss one and you open yourself
-to possible standards-compliance issues.</p>
-
-<p>Furthermore, this position becomes untenable when a single web page must hold
-multiple portions of user-submitted content. Since there's obviously no way
-to find out before-hand what IDs users will use, the blacklist is helpless.
-And since HTML Purifier validates each segment separately, perhaps doing
-so at different times, it would be extremely difficult to dynamically update
-the blacklist in between runs.</p>
-
-<p>Finally, simply destroying the ID is extremely un-userfriendly behavior: after
-all, they might have simply specified a duplicate ID by accident.</p>
-
-<p>Thus, we get to our second method.</p>
-
-
-
-<h2 class="subtitled">Namespacing IDs</h2>
-<div class="subsubtitle">Lazy developer's way, but needs user education</div>
-
-<p>This method, too, is quite simple: add a prefix to all user IDs. With this
-code:</p>
-
-<pre>$config-&gt;set('Attr.EnableID', true);
-$config-&gt;set('Attr.IDPrefix', 'user_');</pre>
-
-<p>...this:</p>
-
-<pre>&lt;a id=&quot;foobar&quot;&gt;Anchor!&lt;/a&gt;</pre>
-
-<p>...turns into:</p>
-
-<pre>&lt;a id=&quot;user_foobar&quot;&gt;Anchor!&lt;/a&gt;</pre>
-
-<p>As long as you don't have any IDs that start with user_, collisions are
-guaranteed not to happen. The drawback is obvious: if a user submits
-id=&quot;foobar&quot;, they probably expect to be able to reference their page with
-#foobar. You'll have to tell them, &quot;No, that doesn't work, you have to add
-user_ to the beginning.&quot;</p>
-
-<p>And yes, things get hairier. Even with a nice prefix, we still have done
-nothing about multiple HTML Purifier outputs on one page. Thus, we have
-a second configuration value to piggy-back off of: %Attr.IDPrefixLocal:</p>
-
-<pre>$config-&gt;set('Attr.IDPrefixLocal', 'comment' . $id . '_');</pre>
-
-<p>This new attributes does nothing but append on to regular IDPrefix, but is
-special in that it is volatile: it's value is determined at run-time and
-cannot possibly be cordoned into, say, a .ini config file. As for what to
-put into the directive, is up to you, but I would recommend the ID number
-the text has been assigned in the database. Whatever you pick, however, it
-has to be unique and stable for the text you are validating. Note, however,
-that we require that %Attr.IDPrefix be set before you use this directive.</p>
-
-<p>And also remember: the user has to know what this prefix is too!</p>
-
-
-
-<h2>Abstinence</h2>
-
-<p>You may not want to bother. That's okay too, just don't enable IDs.</p>
-
-<p>Personally, I would take this road whenever user-submitted content would be
-possibly be shown together on one page. Why a blog comment would need to use
-anchors is beyond me.</p>
-
-
-
-<h2>Denial</h2>
-
-<p>To revert back to pre-1.2.0 behavior, simply:</p>
-
-<pre>$config-&gt;set('Attr.EnableID', true);</pre>
-
-<p>Don't come crying to me when your page mysteriously stops validating, though.</p>
-
-</body>
-</html>
-
-<!-- vim: et sw=4 sts=4
--->