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<title>Advanced API - HTML Purifier</title>
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<h1>Advanced API</h1>
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<div id="home"><a href="http://hp.jpsband.org/">HTML Purifier</a> End-User Documentation</div>
<p>It makes no sense to adopt a <q>one-size-fits-all</q> approach to
filtersets: therefore, users must be able to define their own sets of
<q>allowed</q> elements, as well as switch in-between doctypes of HTML.</p>
<p>Our goals are to let the user:</p>
<dl>
<dt>Select</dt>
<dd><ul>
<li>Doctype</li>
<li>Filtersets: Rich / Plain / Full ...</li>
<li>Mode: Lenient / Correctional</li>
<li>Collections (?): Safe / Unsafe</li>
<li>Modules / Tags / Attributes</li>
</ul></dd>
<dt>Customize</dt>
<dd><ul>
<li>Tags / Attributes / Attribute Types</li>
<li>Filtersets</li>
<li>Root Node</li>
</ul></dd>
<dt>Create</dt>
<dd><ul>
<li>Modules / Tags / Attributes / Attribute Types</li>
<li>Filtersets</li>
<li>Doctype</li>
</ul></dd>
</dl>
<h2>Select</h2>
<h3>Selecting a Doctype</h3>
<p>By default, users will use a doctype-based, permissive but secure
whitelist. They must define a <strong>doctype</strong>, and this serves
as the first method of determining a filterset.</p>
<p class="technical">This identifier is based
on the name the W3C has given to the document type and <em>not</em>
the DTD identifier.</p>
<p>This parameter is set via the configuration object:</p>
<pre>$config->set('HTML', 'Doctype', 'XHTML 1.0 Transitional');</pre>
<h3>Selecting a Filterset</h3>
<p>However, selecting this doctype doesn't mean much, because if we
adhered exactly to the definition we would be letting XSS and other
nasties through. HTML Purifier must, in its filterset, allow a subset
of the doctype, which we shall call a <strong>filterset</strong>.</p>
<p>By default, HTML Purifier will use the <strong>Rich</strong>
filterset, which allows as many elements as possible with untrusted
sources. Other possible filtersets could be:</p>
<dl>
<dt>Full</dt>
<dd>Allows the full span of elements in the doctype, good if you want
HTML Purifier to work as a Tidy substitute but not to strip
anything out.</dd>
<dt>Plain</dt>
<dd>Provides a minimum set of tags for semantic markup of things
like blog comments.</dd>
</dl>
<p>Extension-authors would be able to define custom filtersets for
other users to use.</p>
<p>A possible call to select a filterset would be:</p>
<pre>$config->set('HTML', 'Filterset', 'Rich');</pre>
<h3>Selecting Mode</h3>
<p>Within filtersets, there are various <strong>modes</strong> of operation.
These indicate variant behaviors that, while not strictly changing the
allowed set of elements and attributes, will definitely affect the output.
Currently, we have two modes, which may be used together:</p>
<dl>
<dt>Lenient</dt>
<dd>Deprecated elements and attributes will be transformed into
standards-compliant alternatives when explicitly disallowed. For
example, in the XHTML 1.0 Strict doctype, a <code>center</code>
tag would be turned into a <code>div</code> with the CSS property
<code>text-align:center;</code>, but in XHTML 1.0 Transitional
the tag would be preserved. This mode is on by default.</dd>
<dt>Correctional</dt>
<dd>Deprecated elements and attributes will be transformed into
standards-compliant alternatives whenever possible. Referring
back to the previous example, the <code>center</code> tag would
be transformed in both cases. However, tags without a
reasonable standards-compliant alternative will be preserved
in their form. This mode is on by default. It may have
various levels of operation.</dd>
</dl>
<p>A possible call to select modes would be:</p>
<pre>$config->set('HTML', 'Mode', array('correctional', 'lenient'));</pre>
<p>If modes have extra parameters, a hash might work well:</p>
<pre>$config->set('HTML', 'Mode', array(
'correctional' => 9, // strongest level
'lenient' => true // this one's just boolean
));</pre>
<p>Modes may possibly be wrapped up with the filterset declaration:</p>
<pre>$config->set('HTML', 'Filterset', 'Rich: correctional, lenient');</pre>
<p>Further investigation in this field is necessary.</p>
<h3>Selecting Modules / Tags / Attributes</h3>
<p>If this cookie cutter approach doesn't appeal to a user, they may
decide to roll their own filterset by selecting modules, tags and
attributes to allow.</p>
<p class="technical">This would make use of the same facilities
as a filterset author would use, except that it would go under an
<q>anonymous</q> filterset that would be auto-selected if any of the
relevant module/tag/attribute selection configuration directives were
non-null.</p>
<p>On the highest level, a user will usually be most interested in
directly specifying which elements and attributes are desired. For
example:</p>
<pre>$config->set('HTML', 'AllowedElements', 'a,b,em,p,blockquote,code,i');</pre>
<p>Attribute declarations could be merged into this declaration as such:</p>
<pre>$config->set('HTML', 'Allowed', 'a[href,title],b,em,p[class],blockquote[cite],code,i');</pre>
<p>...or be kept separate:</p>
<pre>$config->set('HTML', 'AllowedAttributes', 'a.href,a.title,p.class,blockquote.cite');</pre>
<p class="technical">Considering that, internally speaking, as mandated by
the XHTML 1.1 Modularization specification, we have organized our
elements around modules, considerable gymnastics will be needed to
get this sort of functionality working.</p>
<p>A user may also specify a module to load a class of elements and attributes
into their filterest:</p>
<pre>$config->set('HTML', 'Allowed', 'Hypertext,Core');</pre>
<p class="fixme">The granularity of these modules is too coarse for
the average user (for example, the core module loads everything from
the essential <code>p</code> tag to the not-so-safe <code>h1</code>
tag). How do we make this still a viable solution?</p>
<h3>Unified selector</h3>
<p>Because selecting each and every one of these configuration options
is a chore, we may wish to offer a specialized configuration method
for selecting a filterset. Possibility:</p>
<pre>function selectFilter($doctype, $filterset, $mode)</pre>
<p>...which is simply a light wrapper over the individual configuration
calls. A custom config file format or text format could also be adopted.</p>
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