HTML Purifier is a Swiss-Army Knife
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HTML Purifier End-User Documentation

HTML Purifier has this quirk where if you try to allow certain elements or attributes, HTML Purifier will tell you that it's not supported, and that you should go to the forums to find out how to implement it. Well, this document is how to implement elements and attributes which HTML Purifier doesn't support out of the box.

Is it necessary?

Before we even write any code, it is paramount to consider whether or not the code we're writing is necessary or not. HTML Purifier, by default, contains a large set of elements and attributes: large enough so that any element or attribute in XHTML 1.0 or 1.1 (and its HTML variants) that can be safely used by the general public is implemented.

So what needs to be implemented? (Feel free to skip this section if you know what you want).


All of the modules listed below are based off of the modularization of XHTML, which, while technically for XHTML 1.1, is quite a useful resource.

If you don't recognize it, you probably don't need it. But the curious can look all of these modules up in the above-mentioned document. Note that inline scripting comes packaged with HTML Purifier (more on this later).


As of HTMLPurifier 2.1.0, we have implemented the Ruby module, which defines a set of tags for publishing short annotations for text, used mostly in Japanese and Chinese school texts, but applicable for positioning any text (not limited to translations) above or below other corresponding text.


HTML 5 is a fork of HTML 4.01 by WHATWG, who believed that XHTML 2.0 was headed in the wrong direction. It too is a working draft, and may change drastically before publication, but it should be noted that the canvas tag has been implemented by many browser vendors.


There are a number of proprietary tags still in the wild. Many of them have been documented in ref-proprietary-tags.txt, but there is currently no implementation for any of them.


There are also a number of other XML languages out there that can be embedded in HTML documents: two of the most popular are MathML and SVG, and I frequently get requests to implement these. But they are expansive, comprehensive specifications, and it would take far too long to implement them correctly (most systems I've seen go as far as whitelisting tags and no further; come on, what about nesting!)

Word of warning: HTML Purifier is currently not namespace aware.

Giving back

As you may imagine from the details above (don't be abashed if you didn't read it all: a glance over would have done), there's quite a bit that HTML Purifier doesn't implement. Recent architectural changes have allowed HTML Purifier to implement elements and attributes that are not safe! Don't worry, they won't be activated unless you set %HTML.Trusted to true, but they certainly help out users who need to put, say, forms on their page and don't want to go through the trouble of reading this and implementing it themself.

So any of the above that you implement for your own application could help out some other poor sap on the other side of the globe. Help us out, and send back code so that it can be hammered into a module and released with the core. Any code would be greatly appreciated!

And now...

Enough philosophical talk, time for some code:

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'enduser-customize.html tutorial');
$config->set('HTML.DefinitionRev', 1);
if ($def = $config->maybeGetRawHTMLDefinition()) {
    // our code will go here

Assuming that HTML Purifier has already been properly loaded (hint: include, this code will set up the environment that you need to start customizing the HTML definition. What's going on?

Turn off caching

To make development easier, we're going to temporarily turn off definition caching:

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'enduser-customize.html tutorial');
$config->set('HTML.DefinitionRev', 1);
$config->set('Cache.DefinitionImpl', null); // TODO: remove this later!
$def = $config->getHTMLDefinition(true);

A few things should be mentioned about the caching mechanism before we move on. For performance reasons, HTML Purifier caches generated HTMLPurifier_Definition objects in serialized files stored (by default) in library/HTMLPurifier/DefinitionCache/Serializer. A lot of processing is done in order to create these objects, so it makes little sense to repeat the same processing over and over again whenever HTML Purifier is called.

In order to identify a cache entry, HTML Purifier uses three variables: the library's version number, the value of %HTML.DefinitionRev and a serial of relevant configuration. Whenever any of these changes, a new HTML definition is generated. Notice that there is no way for the definition object to track changes to customizations: here, it is up to you to supply appropriate information to DefinitionID and DefinitionRev.

Add an attribute

For this example, we're going to implement the target attribute found on a elements. To implement an attribute, we have to ask a few questions:

  1. What element is it found on?
  2. What is its name?
  3. Is it required or optional?
  4. What are valid values for it?

The first three are easy: the element is a, the attribute is target, and it is not a required attribute. (If it was required, we'd need to append an asterisk to the attribute name, you'll see an example of this in the addElement() example).

The last question is a little trickier. Lets allow the special values: _blank, _self, _target and _top. The form of this is called an enumeration, a list of valid values, although only one can be used at a time. To translate this into code form, we write:

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'enduser-customize.html tutorial');
$config->set('HTML.DefinitionRev', 1);
$config->set('Cache.DefinitionImpl', null); // remove this later!
$def = $config->getHTMLDefinition(true);
$def->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');

The Enum#_blank,_self,_target,_top does all the magic. The string is split into two parts, separated by a hash mark (#):

  1. The first part is the name of what we call an AttrDef
  2. The second part is the parameter of the above-mentioned AttrDef

If that sounds vague and generic, it's because it is! HTML Purifier defines an assortment of different attribute types one can use, and each of these has their own specialized parameter format. Here are some of the more useful ones:

Type Format Description
Enum [s:]value1,value2,... Attribute with a number of valid values, one of which may be used. When s: is present, the enumeration is case sensitive.
Bool attribute_name Boolean attribute, with only one valid value: the name of the attribute.
CDATA Attribute of arbitrary text. Can also be referred to as Text (the specification makes a semantic distinction between the two).
ID Attribute that specifies a unique ID
Pixels Attribute that specifies an integer pixel length
Length Attribute that specifies a pixel or percentage length
NMTOKENS Attribute that specifies a number of name tokens, example: the class attribute
URI Attribute that specifies a URI, example: the href attribute
Number Attribute that specifies an positive integer number

For a complete list, consult library/HTMLPurifier/AttrTypes.php; more information on attributes that accept parameters can be found on their respective includes in library/HTMLPurifier/AttrDef.

Sometimes, the restrictive list in AttrTypes just doesn't cut it. Don't sweat: you can also use a fully instantiated object as the value. The equivalent, verbose form of the above example is:

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'enduser-customize.html tutorial');
$config->set('HTML.DefinitionRev', 1);
$config->set('Cache.DefinitionImpl', null); // remove this later!
$def = $config->getHTMLDefinition(true);
$def->addAttribute('a', 'target', new HTMLPurifier_AttrDef_Enum(

Trust me, you'll learn to love the shorthand.

Add an element

Adding attributes is really small-fry stuff, though, and it was possible to add them (albeit a bit more wordy) prior to 2.0. The real gem of the Advanced API is adding elements. There are five questions to ask when adding a new element:

  1. What is the element's name?
  2. What content set does this element belong to?
  3. What are the allowed children of this element?
  4. What attributes does the element allow that are general?
  5. What attributes does the element allow that are specific to this element?

It's a mouthful, and you'll be slightly lost if your not familiar with the HTML specification, so let's explain them step by step.

Content set

The HTML specification defines two major content sets: Inline and Block. Each of these content sets contain a list of elements: Inline contains things like span and b while Block contains things like div and blockquote.

These content sets amount to a macro mechanism for HTML definition. Most elements in HTML are organized into one of these two sets, and most elements in HTML allow elements from one of these sets. If we had to write each element verbatim into each other element's allowed children, we would have ridiculously large lists; instead we use content sets to compactify the declaration.

Practically speaking, there are several useful values you can use here:

Content set Description
Inline Character level elements, text
Block Block-like elements, like paragraphs and lists
false Any element that doesn't fit into the mold, for example li or tr

By specifying a valid value here, all other elements that use that content set will also allow your element, without you having to do anything. If you specify false, you'll have to register your element manually.

Allowed children

Allowed children defines the elements that this element can contain. The allowed values may range from none to a complex regexp depending on your element.

If you've ever taken a look at the HTML DTD's before, you may have noticed declarations like this:

<!ELEMENT LI - O (%flow;)*             -- list item -->

The (%flow;)* indicates the allowed children of the li tag: li allows any number of flow elements as its children. (The - O allows the closing tag to be omitted, though in XML this is not allowed.) In HTML Purifier, we'd write it like Flow (here's where the content sets we were discussing earlier come into play). There are three shorthand content models you can specify:

Content model Description
Empty No children allowed, like br or hr
Inline Any number of inline elements and text, like span
Flow Any number of inline elements, block elements and text, like div

This covers 90% of all the cases out there, but what about elements that break the mold like ul? This guy requires at least one child, and the only valid children for it are li. The content model is: Required: li. There are two parts: the first type determines what ChildDef will be used to validate content models. The most common values are:

Type Description
Required Children must be one or more of the valid elements
Optional Children can be any number of the valid elements
Custom Children must follow the DTD-style regex

You can also implement your own ChildDef: this was done for a few special cases in HTML Purifier such as Chameleon (for ins and del), StrictBlockquote and Table.

The second part specifies either valid elements or a regular expression. Valid elements are separated with horizontal bars (|), i.e. "a | b | c". Use #PCDATA to represent plain text. Regular expressions are based off of DTD's style:

For example, "a, b?, (c | d), e+, f*" means "In this order, one a element, at most one b element, one c or d element (but not both), one or more e elements, and any number of f elements." Regex veterans should be able to jump right in, and those not so savvy can always copy-paste W3C's content model definitions into HTML Purifier and hope for the best.

A word of warning: while the regex format is extremely flexible on the developer's side, it is quite unforgiving on the user's side. If the user input does not exactly match the specification, the entire contents of the element will be nuked. This is why there is are specific content model types like Optional and Required: while they could be implemented as Custom: (valid | elements)*, the custom classes contain special recovery measures that make sure as much of the user's original content gets through. HTML Purifier's core, as a rule, does not use Custom.

One final note: you can also use Content Sets inside your valid elements lists or regular expressions. In fact, the three shorthand content models mentioned above are just that: abbreviations:

Content model Implementation
Inline Optional: Inline | #PCDATA
Flow Optional: Flow | #PCDATA

When the definition is compiled, Inline will be replaced with a horizontal-bar separated list of inline elements. Also, notice that it does not contain text: you have to specify that yourself.

Common attributes

Congratulations: you have just gotten over the proverbial hump (Allowed children). Common attributes is much simpler, and boils down to one question: does your element have the id, style, class, title and lang attributes? If so, you'll want to specify the Common attribute collection, which contains these five attributes that are found on almost every HTML element in the specification.

There are a few more collections, but they're really edge cases:

Collection Attributes
I18N lang, possibly xml:lang
Core style, class, id and title

Common is a combination of the above-mentioned collections.

Readers familiar with the modularization may have noticed that the Core attribute collection differs from that specified by the abstract modules of the XHTML Modularization 1.1. We believe this section to be in error, as br permits the use of the style attribute even though it uses the Core collection, and the DTD and XML Schemas supplied by W3C support our interpretation.


If you didn't read the earlier section on adding attributes, read it now. The last parameter is simply an array of attribute names to attribute implementations, in the exact same format as addAttribute().

Putting it all together

We're going to implement form. Before we embark, lets grab a reference implementation from over at the transitional DTD:

<!ELEMENT FORM - - (%flow;)* -(FORM)   -- interactive form -->
  %attrs;                              -- %coreattrs, %i18n, %events --
  action      %URI;          #REQUIRED -- server-side form handler --
  method      (GET|POST)     GET       -- HTTP method used to submit the form--
  enctype     %ContentType;  "application/x-www-form-urlencoded"
  accept      %ContentTypes; #IMPLIED  -- list of MIME types for file upload --
  name        CDATA          #IMPLIED  -- name of form for scripting --
  onsubmit    %Script;       #IMPLIED  -- the form was submitted --
  onreset     %Script;       #IMPLIED  -- the form was reset --
  target      %FrameTarget;  #IMPLIED  -- render in this frame --
  accept-charset %Charsets;  #IMPLIED  -- list of supported charsets --

Juicy! With just this, we can answer four of our five questions:

  1. What is the element's name? form
  2. What content set does this element belong to? Block (this needs a little sleuthing, I find the easiest way is to search the DTD for FORM and determine which set it is in.)
  3. What are the allowed children of this element? One or more flow elements, but no nested forms
  4. What attributes does the element allow that are general? Common
  5. What attributes does the element allow that are specific to this element? A whole bunch, see ATTLIST; we're going to do the vital ones: action, method and name

Time for some code:

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'enduser-customize.html tutorial');
$config->set('HTML.DefinitionRev', 1);
$config->set('Cache.DefinitionImpl', null); // remove this later!
$def = $config->getHTMLDefinition(true);
$def->addAttribute('a', 'target', new HTMLPurifier_AttrDef_Enum(
$form = $def->addElement(
  'form',   // name
  'Block',  // content set
  'Flow', // allowed children
  'Common', // attribute collection
  array( // attributes
    'action*' => 'URI',
    'method' => 'Enum#get|post',
    'name' => 'ID'
$form->excludes = array('form' => true);

Each of the parameters corresponds to one of the questions we asked. Notice that we added an asterisk to the end of the action attribute to indicate that it is required. If someone specifies a form without that attribute, the tag will be axed. Also, the extra line at the end is a special extra declaration that prevents forms from being nested within each other.

And that's all there is to it! Implementing the rest of the form module is left as an exercise to the user; to see more examples check the library/HTMLPurifier/HTMLModule/ directory in your local HTML Purifier installation.

And beyond...

Perceptive users may have realized that, to a certain extent, we have simply re-implemented the facilities of XML Schema or the Document Type Definition. What you are seeing here, however, is not just an XML Schema or Document Type Definition: it is a fully expressive method of specifying the definition of HTML that is a portable superset of the capabilities of the two above-mentioned schema languages. What makes HTMLDefinition so powerful is the fact that if we don't have an implementation for a content model or an attribute definition, you can supply it yourself by writing a PHP class.

There are many facets of HTMLDefinition beyond the Advanced API I have walked you through today. To find out more about these, you can check out these source files:

Notes for HTML Purifier 4.2.0 and earlier

Previously, this tutorial gave some incorrect template code for editing raw definitions, and that template code will now produce the error Due to a documentation error in previous version of HTML Purifier... Here is how to mechanically transform old-style code into new-style code.

First, identify all code that edits the raw definition object, and put it together. Ensure none of this code must be run on every request; if some sub-part needs to always be run, move it outside this block. Here is an example below, with the raw definition object code bolded.

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'enduser-customize.html tutorial');
$config->set('HTML.DefinitionRev', 1);
$def = $config->getHTMLDefinition(true);
$def->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');
$purifier = new HTMLPurifier($config);

Next, replace the raw definition retrieval with a maybeGetRawHTMLDefinition method call inside an if conditional, and place the editing code inside that if block.

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'enduser-customize.html tutorial');
$config->set('HTML.DefinitionRev', 1);
if ($def = $config->maybeGetRawHTMLDefinition()) {
    $def->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');
$purifier = new HTMLPurifier($config);

And you're done! Alternatively, if you're OK with not ever caching your code, the following will still work and not emit warnings.

$config = HTMLPurifier_Config::createDefault();
$def = $config->getHTMLDefinition(true);
$def->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');
$purifier = new HTMLPurifier($config);

A slightly less efficient version of this was what was going on with old versions of HTML Purifier.

Technical notes: ajh pointed out on in a forum topic that HTML Purifier appeared to be repeatedly writing to the cache even when a cache entry already existed. Investigation lead to the discovery of the following infelicity: caching of customized definitions didn't actually work! The problem was that even though a cache file would be written out at the end of the process, there was no way for HTML Purifier to say, Actually, I've already got a copy of your work, no need to reconfigure your customizations. This required the API to change: placing all of the customizations to the raw definition object in a conditional which could be skipped.