HTML Purifier
  by Edward Z. Yang

There are a number of ad hoc HTML filtering solutions out there on the web
(some examples including HTML_Safe, kses and SafeHtmlChecker.class.php) that
claim to filter HTML properly, preventing malicious JavaScript and layout
breaking HTML from getting through the parser.  None of them, however,
demonstrates a thorough knowledge of neither the DTD that defines the HTML
nor the caveats of HTML that cannot be expressed by a DTD.  Configurable
filters (such as kses or PHP's built-in striptags() function) have trouble
validating the contents of attributes and can be subject to security attacks
due to poor configuration.  Other filters take the naive approach of
blacklisting known threats and tags, failing to account for the introduction
of new technologies, new tags, new attributes or quirky browser behavior.

However, HTML Purifier takes a different approach, one that doesn't use
specification-ignorant regexes or narrow blacklists.  HTML Purifier will
decompose the whole document into tokens, and rigorously process the tokens by:
removing non-whitelisted elements, transforming bad practice tags like <font>
into <span>, properly checking the nesting of tags and their children and
validating all attributes according to their RFCs.

To my knowledge, there is nothing like this on the web yet.  Not even MediaWiki,
which allows an amazingly diverse mix of HTML and wikitext in its documents,
gets all the nesting quirks right.  Existing solutions hope that no JavaScript
will slip through, but either do not attempt to ensure that the resulting
output is valid XHTML or send the HTML through a draconic XML parser (and yet
still get the nesting wrong: SafeHtmlChecker.class.php does not prevent <a>
tags from being nested within each other).

This document no longer is a detailed description of how HTMLPurifier works,
as those descriptions have been moved to the appropriate code.  The first
draft was drawn up after two rough code sketches and the implementation of a
forgiving lexer.  You may also be interested in the unit tests located in the
tests/ folder, which provide a living document on how exactly the filter deals
with malformed input.

In summary (see corresponding classes for more details):

1. Parse document into an array of tag and text tokens (Lexer)
2. Remove all elements not on whitelist and transform certain other elements
   into acceptable forms (i.e. <font>)
3. Make document well formed while helpfully taking into account certain quirks,
   such as the fact that <p> tags traditionally are closed by other block-level
   elements.
4. Run through all nodes and check children for proper order (especially
   important for tables).
5. Validate attributes according to more restrictive definitions based on the
   RFCs.
6. Translate back into a string. (Generator)

HTML Purifier is best suited for documents that require a rich array of
HTML tags.  Things like blog comments are, in all likelihood, most appropriately
written in an extremely restrictive set of markup that doesn't require
all this functionality (or not written in HTML at all), although this may
be changing in the future with the addition of levels of filtering.