Customize!

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).

XHTML 1.0

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.

• Structure
• Frames
• Applets (deprecated)
• Forms
• Image maps
• Objects
• Frames
• Events
• Meta-information
• Style sheets
• Link (not hypertext)
• Base
• Name

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).

XHTML 1.1

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.

XHTML 2.0

XHTML 2.0 is still a working draft, so any elements introduced in the specification have not been implemented and will not be implemented until we get a recommendation or proposal. Because XHTML 2.0 is an entirely new markup language, implementing rules for it will be no easy task.

HTML 5

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.

Proprietary

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.

Extensions

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);
$def = $config->getHTMLDefinition(true);

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

• The first three lines are regular configuration code:
  • %HTML.DefinitionID is set to a unique identifier for your custom HTML definition. This prevents it from clobbering other custom definitions on the same installation.
  • %HTML.DefinitionRev is a revision integer of your HTML definition. Because HTML definitions are cached, you'll need to increment this whenever you make a change in order to flush the cache.
• The fourth line retrieves a raw HTMLPurifier_HTMLDefinition object that we will be tweaking. If the parameter was removed, we would be retrieving a fully formed definition object, which is somewhat useless for customization purposes.

Broken backwards-compatibility

Those of you who have already been twiddling around with the raw HTML definition object, you'll be noticing that you're getting an error when you attempt to retrieve the raw definition object without specifying a DefinitionID. It is vital to caching (see below) that you make a unique name for your customized definition, so make up something right now and things will operate again.

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:

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(
  array('_blank','_self','_target','_top')
));

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:

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:

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:

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:

• Parentheses () are used for grouping
• Commas (,) separate elements that should come one after another
• Horizontal bars (|) indicate one or the other elements should be used
• Plus signs (+) are used for a one or more match
• Asterisks (*) are used for a zero or more match
• Question marks (?) are used for a zero or one match

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:

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:

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.

Attributes

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 -->
<!ATTLIST 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(
  array('_blank','_self','_target','_top')
));
$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:

• library/HTMLPurifier/HTMLModule.php
• library/HTMLPurifier/ElementDef.php