htmLawed documentation

1  About htmLawed
  1.1  Example uses
  1.2  Features
  1.3  History
  1.4  License & copyright
  1.5  Terms used here
  1.6  Availability
2  Usage
  2.1  Simple
  2.2  Configuring htmLawed using the $config argument
  2.3  Extra HTML specifications using the $spec argument
  2.4  Performance time & memory usage
  2.5  Some security risks to keep in mind
  2.6  Use with kses() code
  2.7  Tolerance for ill-written HTML
  2.8  Limitations & work-arounds
  2.9  Examples of usage
3  Details
  3.1  Invalid/dangerous characters
  3.2  Character references/entities
  3.3  HTML elements
    3.3.1  HTML comments & CDATA sections
    3.3.2  Tag-transformation for better compliance with standards
    3.3.3  Tag balancing & proper nesting
    3.3.4  Elements requiring child elements
    3.3.5  Beautify or compact HTML
    3.3.6  Custom elements
  3.4  Attributes
    3.4.1  Auto-addition of XHTML-required attributes
    3.4.2  Duplicate/invalid id values
    3.4.3  URL schemes & scripts in attribute values
    3.4.4  Absolute & relative URLs
    3.4.5  Lower-cased, standard attribute values
    3.4.6  Transformation of deprecated attributes
    3.4.7  Anti-spam & href
    3.4.8  Inline style properties
    3.4.9  Hook function for tag content
  3.5  Simple configuration directive for most valid XHTML
  3.6  Simple configuration directive for most safe HTML
  3.7  Using a hook function
  3.8  Obtaining finalized parameter values
  3.9  Retaining non-HTML tags in input with mixed markup
4  Other
  4.1  Support
  4.2  Known issues
  4.3  Change-log
  4.4  Testing
  4.5  Upgrade, & old versions
  4.6  Comparison with HTMLPurifier
  4.7  Use through application plug-ins/modules
  4.8  Use in non-PHP applications
  4.9  Donate
  4.10  Acknowledgements
5  Appendices
  5.1  Characters discouraged in HTML
  5.2  Valid attribute-element combinations
  5.3  CSS 2.1 properties accepting URLs
  5.4  Microsoft Windows 1252 character replacements
  5.5  URL format
  5.6  Brief on htmLawed code

htmLawed_README.txt, 4 August 2023
htmLawed 1.2.15
Copyright Santosh Patnaik
Dual licensed with LGPL 3 and GPL 2+
A PHP Labware internal utility - 

1  About htmLawed

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  htmLawed is a PHP script to process text with HTML markup to make it more compliant with HTML standards and with administrative policies. It works by making HTML well-formed with balanced and properly nested tags, neutralizing code that introduces a security vulnerability or is used for cross-site scripting (XSS) attacks, allowing only specified HTML tags and attributes, and so on. Such lawing in of HTML code ensures that it is in accordance with the aesthetics, safety and usability requirements set by administrators.

  htmLawed is highly customizable, and fast with low memory usage. Its free and open-source code is in one small file. It does not require extensions or libraries, and works in older versions of PHP as well. It is a good alternative to the HTML Tidy application.

1.1  Example uses

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  *  Filtering of text submitted as comments on blogs to allow only certain HTML elements

  *  Making RSS newsfeed items standard-compliant: often one uses an excerpt from an HTML document for the content, and with unbalanced tags, non-numerical entities, etc., such excerpts may not be XML-compliant

  *  Beautifying or pretty-printing HTML code

  *  Text processing for stricter XML standard-compliance: e.g., to have lowercased x in hexadecimal numeric entities becomes necessary if an HTML document with MathML content needs to be served as application/xml

  *  Scraping text from web-pages

  *  Transforming an HTML element to another

1.2  Features

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  Key: * security feature, ^ standard compliance, ~ requires setting right options


  *  makes input more secure and standard-compliant for HTML as well as generic XML documents  ^
  *  supports markup for HTML 5, custom elements, and microdata, ARIA, Ruby, custom attributes, etc.  ^
  *  can beautify or compact HTML  ~
  *  works with input of almost any character encoding and does not affect it
  *  has good tolerance for ill-written HTML

  *  can enforce restricted use of elements  *~
  *  ensures proper closure of empty elements like img  ^
  *  transforms deprecated elements like font  ^~
  *  can permit HTML comments and CDATA sections  ^~
  *  can permit all elements, including script, object and form  ~

  *  can restrict attributes by element  ^~
  *  removes invalid attributes  ^
  *  lower-cases element and attribute names  ^
  *  provides required attributes, like alt for image  ^
  *  transforms deprecated attributes  ^~
  *  ensures attributes are declared only once  ^
  *  permits custom, non-standard attributes as well as custom rules for standard attributes  ~

  *  declares value for empty (minimized or boolean) attributes like checked  ^
  *  checks for potentially dangerous attribute values  *~
  *  ensures unique id attribute values  ^~
  *  double-quotes attribute values  ^
  *  lower-cases standard attribute values like password  ^

  *  can restrict URL protocol/scheme by attribute  *~
  *  can disable dynamic expressions in style values  *~

  *  neutralizes invalid named character entities  ^
  *  converts hexadecimal numeric entities to decimal ones, or vice versa  ^~
  *  converts named entities to numeric ones for generic XML use  ^~

  *  removes null characters  *
  *  neutralizes potentially dangerous proprietary Netscape Javascript entities  *
  *  replaces potentially dangerous soft-hyphen character in URL-accepting attribute values with spaces  *

  *  removes common invalid characters not allowed in HTML or XML  ^
  *  replaces characters from Microsoft applications like Word that are discouraged in HTML or XML  ^~
  *  neutralize entities for characters invalid or discouraged in HTML or XML  ^
  *  appropriately neutralize <, &, ", and > characters  ^*

  *  understands improperly spaced tag content (e.g., spread over more than a line) and properly spaces them
  *  attempts to balance tags for well-formedness  ^~
  *  understands when omitable closing tags like </p> are missing  ^~
  *  attempts to permit only validly nested tags  ^~
  *  can either remove or neutralize bad content ^~
  *  attempts to rectify common errors of plain-text misplacement (e.g., directly inside blockquote) ^~

  *  has optional anti-spam measures such as addition of rel="nofollow" and link-disabling  ~
  *  optionally makes relative URLs absolute, and vice versa  ~

  *  optionally marks & to identify the entities for &, < and > introduced by it  ~

  *  allows deployment of powerful hook functions to inject HTML, consolidate style attributes to class, finely check attribute values, etc.  ~

1.3  History

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  htmLawed was created in 2007 for use with LabWiki, a wiki software developed at PHP Labware, as a suitable software could not be found. Existing PHP software like Kses and HTMLPurifier were deemed inadequate, slow, resource-intensive, or dependent on an extension or external application like HTML Tidy. The core logic of htmLawed, that of identifying HTML elements and attributes, was based on the Kses (version 0.2.2) HTML filter software of Ulf Harnhammar (it can still be used with code that uses Kses; see section 2.6.). Support for HTML version 5 was added in May 2013 in a beta and in February 2017 in a production release.

  See section 4.3 for a detailed log of changes in htmLawed over the years, and section 4.10 for acknowledgements.

1.4  License & copyright

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  htmLawed is free and open-source software, copyrighted by Santosh Patnaik, MD, PhD, and dual-licensed with LGPL version 3, and GPL version 2 (or later) licenses.

1.5  Terms used here

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  In this document, only HTML body-level elements are considered. htmLawed does not have support for head-level elements, body, and the frame-level elements, frameset, frame and noframes, and these elements are ignored here.

  *  administrator - or admin; person setting up the code that utilizes htmLawed; also, user
  *  attributes - name-value pairs like href="" in opening tags
  *  author - see writer
  *  character - atomic unit of text; internally represented by a numeric code-point as specified by the encoding or charset in use
  *  entity - markup like &gt; and &#160; used to refer to a character
  *  element - HTML element like a and img
  *  element content -  content between the opening and closing tags of an element, like click of the <a href="x">click</a> element
  *  HTML - implies XHTML unless specified otherwise
  *  HTML body - content in the body container of an HTML document
  *  input - text given to htmLawed to process
  *  legal – standard-compliant; also, valid
  *  processing - involves filtering, correction, etc., of input
  *  safe - absence or reduction of certain characters and HTML elements and attributes in HTML of text that can otherwise potentially, and circumstantially, expose text readers to security vulnerabilities like cross-site scripting attacks (XSS)
  *  scheme - a URL protocol like http and ftp
  *  specification - detailed description including rules that define HTML
  *  standard – widely accepted specification
  *  style property - terms like border and height for which declarations are made in values for the style attribute of elements
  *  tag - markers like <a href="x"> and </a> delineating element content; the opening tag can contain attributes
  *  tag content - consists of tag markers < and >, element names like div, and possibly attributes
  *  user - administrator
  *  valid - see legal
  *  writer - end-user like a blog commenter providing the input that is to be processed; also, author
  *  XHTML - XML-compliant HTML; parsing rules for XHTML are more strict than for regular HTML

1.6  Availability

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  htmLawed can be downloaded for free at its website. Besides the htmLawed.php file, the download has the htmLawed documentation (this document) in plain text and HTML formats, a script for testing, and a text file for test-cases. htmLawed can be installed with Composer, and is also available as a PHP class (OOP code) – see the website. Official htmLawed releases are also put up on Sourceforge.

2  Usage

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  htmLawed works in PHP version 4.4 or higher. Either include() the htmLawed.php file, or copy-paste the entire code.

  To use with PHP 4.3, have the following code included:

     function ctype_digit($var){
      return ((int) $var == $var);

2.1  Simple

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  The input text to be processed, $text, is passed as an argument of type string; htmLawed() returns the processed string:

    $processed = htmLawed($text);

  With the htmLawed class (section 1.6), usage is:

    $processed = htmLawed::hl($text);

  Notes: (1) If input is from a $_GET or $_POST value, and magic quotes are enabled on the PHP setup, run stripslashes() on the input before passing to htmLawed. (2) htmLawed does not have support for head-level elements, body, and the frame-level elements, frameset, frame and noframes.

  By default, htmLawed will process the text allowing all valid HTML elements/tags and commonly used URL schemes and CSS style properties. It will allow Javascript code, CDATA sections and HTML comments, balance tags, and ensure proper nesting of elements. Such actions can be configured using two other optional arguments -- $config and $spec:

    $processed = htmLawed($text, $config, $spec);

  The $config and $spec arguments are detailed below. Some examples are shown in section 2.9. For maximum protection against XSS and other security vulnerabilities, consider using the safe parameter; see section 3.6.

2.2  Configuring htmLawed using the $config argument

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  $config instructs htmLawed on how to tackle certain tasks. When $config is not specified, or not set as an array (e.g., $config = 1), htmLawed will take default actions. One or many of the task-action or parameter-value pairs can be specified in $config as array key-value pairs. If a parameter is not specified, htmLawed will use the default value for it, indicated further below. In PHP code, parameter values that are integers should not be quoted and should be used as numeric types (unless meant as string/text). Thus, for instance:

    $config = array('comment'=>0, 'cdata'=>1, 'elements'=>'a, b, strong');
    $processed = htmLawed($text, $config);

  Below are the various parameters that can be specified in $config.

  Key: * default, ^ different from htmLawed versions below 1.2, ~ different default when valid_xhtml is set to 1 (see section 3.5), " different default when safe is set to 1 (see section 3.6)

  Make URLs absolute or relative; $config["base_url"] needs to be set; see section 3.4.4

  -1 - make relative
  0 - no action  *
  1 - make absolute

  Mark & characters in the original input; see section 3.2

  Anti-link-spam measure; see section 3.4.7

  0 - no measure taken  *
  array("regex1", "regex2") - will ensure a rel attribute with nofollow in its value in case the href attribute value matches the regular expression pattern regex1, and/or will remove href if its value matches the regular expression pattern regex2. E.g., array("/./", "/://\W*(?!(abc\.com|xyz\.org))/"); see section 3.4.7 for more.

  Anti-mail-spam measure; see section 3.4.7

  0 - no measure taken  *
  word - @ in mail address in href attribute value is replaced with specified word

  Permit any custom element; regardless of this setting, specific custom elements can be denied or permitted through $config["elements"]; see section 3.3.6

  0 - no
  1 - yes  *

  Balance tags for well-formedness and proper nesting; see section 3.3.3

  0 - no
  1 - yes  *

  Base URL value that needs to be set if $config["abs_url"] is not 0; see section 3.4.4

  Handling of CDATA sections; see section 3.3.1

  0 - don't consider CDATA sections as markup and proceed as if plain text  "
  1 - remove
  2 - allow, but neutralize any <, >, and & inside by converting them to named entities
  3 - allow  *

  Replace discouraged characters introduced by Microsoft Word, etc.; see section 3.1

  0 - no  *
  1 - yes
  2 - yes, but replace special single & double quotes with ordinary ones

  Handling of HTML comments; see section 3.3.1

  0 - don't consider comments as markup and proceed as if plain text  "
  1 - remove
  2 - allow, but neutralize any <, >, and & inside by converting to named entities
  3 - allow  *

  Allow dynamic CSS expression by not removing the expression from CSS property values in style attributes; see section 3.4.8

  0 - remove  *
  1 - allow

  Denied HTML attributes; see section 3.4

  0 - none  *
  string - dictated by values in string
  on* - on* event attributes like onfocus not allowed  "

  Allow direct nesting of a list within another without requiring it to be a list item; see section 3.3.4

  0 - no  *
  1 - yes

  Allowed HTML elements; see section 3.3

  all - *^
  * -acronym -big -center -dir -font -isindex -s -strike -tt -  ~^
  applet, audio, canvas, dialog, embed, iframe, object, script, and video elements not allowed -  "^

  Allow hexadecimal numeric entities and do not convert to the more widely accepted decimal ones, or convert decimal to hexadecimal ones; see section 3.2

  0 - no
  1 - yes  *
  2 - convert decimal to hexadecimal ones

  Name of an optional hook function to alter the input string, $config or $spec before htmLawed enters the main phase of its work; see section 3.7

  0 - no hook function  *
  name - name is name of the hook function

  Name of an optional hook function to alter tag content finalized by htmLawed; see section 3.4.9

  0 - no hook function  *
  name - name is name of the hook function

  Neutralize bad tags by converting their < and > characters to entities, or remove them; see section 3.3.3

  0 - remove
  1 - neutralize both tags and element content
  2 - remove tags but neutralize element content
  3 and 4 - like 1 and 2 but remove if text (pcdata) is invalid in parent element
  5 and 6 * -  like 3 and 4 but line-breaks, tabs and spaces are left

  For XHTML compliance, predefined, standard attribute values, like get for the method attribute of form, must be lowercased; see section 3.4.5

  0 - no
  1 - yes  *

  Transform or remove these deprecated HTML elements, even if they are allowed by the admin: acronym, applet, big, center, dir, font, isindex, s, strike, tt; see section 3.3.2

  0 - no
  1 - yes, but leave applet and isindex that currently cannot be transformed  *^
  2 - yes, removing applet and isindex elements and their contents (nested elements remain)  ~^

  Allow non-universal named HTML entities, or convert to numeric ones; see section 3.2

  0 - convert
  1 - allow  *

  Allow deprecated attributes or transform them; see section 3.4.6

  0 - allow
  1 - transform, but name attributes for a and map are retained  *
  2 - transform

  Name of the parent element, possibly imagined, that will hold the input; see section 3.3

  Magic parameter to make input the most secure against vulnerabilities like XSS without needing to specify other relevant $config parameters; see section 3.6

  0 - no  *
  1 - will auto-adjust other relevant $config parameters (indicated by " in this list)  ^

  Array of attribute-specific, comma-separated, lower-cased list of schemes (protocols) allowed in attributes accepting URLs (or ! to deny any URL); * covers all unspecified attributes; see section 3.4.3

  href: aim, app, feed, file, ftp, gopher, http, https, javascript, irc, mailto, news, nntp, sftp, ssh, tel, telnet, ws, wss; *:data, file, http, https, javascript, ws, wss  *^
  href: aim, feed, file, ftp, gopher, http, https, irc, mailto, news, nntp, sftp, ssh, tel, telnet, ws, wss; style: !; *:file, http, https, ws, wss  "

  Name of a PHP variable to assign the finalized $config and $spec values; see section 3.8

  Ignore style attribute values, letting them through without any alteration

  0 - no *
  1 - htmLawed will let through any style value; see section 3.4.8

  Beautify or compact HTML code; see section 3.3.5

  -1 - compact
  0 - no  *
  1 or string - beautify (custom format specified by string)

  id attribute value checks; see section 3.4.2

  0 - no
  1 - remove duplicate and/or invalid ones  *
  word - remove invalid ones and replace duplicate ones with new and unique ones based on the word; the admin-specified word cannot contain a space character

  Magic parameter to make input the most valid XHTML without needing to specify other relevant $config parameters; see section 3.5

  0 - no  *
  1 - will auto-adjust other relevant $config parameters (indicated by ~ in this list)

  Auto-add xml:lang attribute; see section 3.4.1

  0 - no  *
  1 - add if lang attribute is present
  2 - add if lang attribute is present, and remove lang  ~

2.3  Extra HTML specifications using the $spec parameter

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  The $spec argument of htmLawed can be used to disallow an otherwise legal attribute for an element, or to restrict the attribute's values. This can also be helpful as a security measure (e.g., in certain versions of browsers, certain values can cause buffer overflows and denial of service attacks), or in enforcing admin policies. $spec is specified as a string of text containing one or more rules, with multiple rules separated from each other by a semi-colon (;). E.g.,

    $spec = 'i=-*; td, tr=style, id, -*; a=id(match="/[a-z][a-z\d.:\-`"]*/i"/minval=2), href(maxlen=100/minlen=34); img=-width,-alt';
    $processed = htmLawed($text, $config, $spec);


    $processed = htmLawed($text, $config, 'i=-*; td, tr=style, id, -*; a=id(match="/[a-z][a-z\d.:\-`"]*/i"/minval=2), href(maxlen=100/minlen=34); img=-width,-alt');

  A rule begins with an HTML element name(s) (rule-element), for which the rule applies, followed by an equal-to (=) sign. A rule-element may represent multiple elements if comma (,)-separated element names are used. E.g., th,td,tr=.

  Rest of the rule consists of comma-separated HTML attribute names, which can be the wildcard references *, aria*, data*, and on* for the sets of all standard, Aria, data-*, and event (on*) attributes, respectively. A minus (-) character before an attribute means that the attribute is not permitted inside the rule-element. E.g., -width. To deny all attributes, -* can be used. All Aria, data-*, and event (on*) attributes can similarly be denined using aria*, data*, and on*, respectively.

  Following shows examples of rule excerpts with rule-element a and the attributes that are being permitted:

  *  a= - all
  *  a=id - all
  *  a=href, title, -id, -onclick - all except id and onclick
  *  a=*, id, -id - all except id
  *  a=-* - none
  *  a=-*, href, title - none except href and title
  *  a=-*, -id, href, title - none except href and title
  *  a=-on*, -id, href, onclick, title - all except id and on* other than onclick

  Rules regarding attribute values are optionally specified inside round brackets after attribute names – which cannot be wildcard references like * or data* – in solidus (/)-separated parameter = value pairs. E.g., title(maxlen=30/minlen=5). None or one or more of the following parameters may be specified:

  *  oneof - one or more choices separated by | that the value should match; if only one choice is provided, then the value must match that choice; matching is case-sensitive

  *  noneof - one or more choices separated by | that the value should not match; matching is case-sensitive

  *  maxlen and minlen - upper and lower limits for the number of characters in the attribute value; specified in numbers

  *  maxval and minval - upper and lower limits for the numerical value specified in the attribute value; specified in numbers

  *  match and nomatch - pattern that the attribute value should or should not match; specified as PHP/PCRE-compatible regular expressions with delimiters and possibly modifiers (e.g., to specify case-sensitivity for matching)

  *  default - a value to force on the attribute if the value provided by the writer does not fit any of the specified parameters

  If default is not set and the attribute value does not satisfy any of the specified parameters, then the attribute is removed. The default value can also be used to force all attribute declarations to take the same value (by getting the values declared illegal by setting, e.g., maxlen to -1).

  Examples with input <input title="WIDTH" value="10em" /><input title="length" value="5" class="ic1 ic2" /> are shown below.

  Rule: input=title(maxlen=60/minlen=6), value
  Output: <input value="10em" /><input title="length" value="5" class="ic1 ic2" />

  Rule: input=title(), value(maxval=8/default=6)
  Output: <input title="WIDTH" value="6" /><input title="length" value="5" class="ic1 ic2" />

  Rule: input=title(nomatch=%w.d%i), value(match=%em%/default=6em)
  Output: <input value="10em" /><input title="length" value="6em" class="ic1 ic2" />

  Rule: input=class(noneof=ic2|ic3/oneof=ic1|ic4), title(oneof=height|depth/default=depth), value(noneof=5|6)
  Output: <input title="depth" value="10em" /><input title="depth" class="ic1" />

  Special characters: The characters ;, ,, /, (, ), |, ~ and space have special meanings in the rules. Words in the rules that use such characters, or the characters themselves, should be escaped by enclosing in pairs of double-quotes ("). A back-tick (`) can be used to escape a literal ". An example rule illustrating this is input=value(maxlen=30/match="/^\w/"/default="your `"ID`"").

  Attributes that accept multiple values: If an attribute is accesskey, class, itemtype or rel, or archive in case of object element, which can have multiple, space-separated values, or archive in case of object element and srcset, which can have multiple, comma-separated values, htmLawed will parse the attribute value for such multiple values and will individually test each of them. The parsing is performed after any URL assessment of the attribute values (section 3.4.3).

  Note: To deny an attribute for all elements for which it is legal, $config["deny_attribute"] (see section 3.4) can be used instead of $spec. Also, attributes can be allowed element-specifically through $spec while being denied globally through $config["deny_attribute"]. The hook_tag parameter (section 3.4.9) can also be possibly used to implement a functionality like that achieved using $spec functionality.

  Note: Attributes permitted through $spec are permitted regardless of any denial through $config. An attribute for which $spec indicates both permission and denial will be permitted. E.g., onclick with $spec value of a = *, -onclick, onclick, a = -on*, onclick or a = on*, -onclick will be permitted inside a.

  Note: Attributes' specifications for an element may be (inadvertently) set through multiple rules. In case of conflict, the attribute specification in the first rule will get precedence.

  $spec can also be used to permit custom or non-standard attributes. Thus, the following value of $spec will permit the custom uses of the standard rel attribute in input (not permitted as per standards) and of a non-standard attribute, vFlag, in img.

    $spec = 'img=vFlag; input=rel'

  The attribute names must begin with an alphabet and cannot have space, equal-to (=) and solidus (/) characters.

2.4  Performance time & memory usage

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  The time and memory consumed during text processing by htmLawed depends on its configuration, the size of the input, and the amount, nestedness and well-formedness of the HTML markup within the input. In particular, tag balancing and beautification each can increase the processing time by about a quarter.

  The htmLawed demo can be used to evaluate the performance and effects of different types of input and $config.

2.5  Some security risks to keep in mind

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  When setting the parameters/arguments (like those to allow certain HTML elements) for use with htmLawed, one should bear in mind that the setting may let through potentially dangerous HTML code which is meant to steal user-data, deface a website, render a page non-functional, etc. Unless end-users, either people or software, supplying the content are completely trusted, security issues arising from the degree of HTML usage permitted through htmLawed's setting should be considered. For example, following increase security risks:

  *  Allowing script, applet, embed, iframe, canvas, audio, video, dialog or object elements, or certain of their attributes like allowscriptaccess

  *  Allowing HTML comments (some Internet Explorer versions are vulnerable with, e.g., <!--[if gte IE 4]><script>alert("xss");</script><![endif]-->

  *  Allowing dynamic CSS expressions (some Internet Explorer versions are vulnerable)

  *  Allowing the style attribute

  To remove unsecure HTML, code-developers using htmLawed must set $config appropriately. E.g., $config["elements"] = "* -script" to deny the script element (section 3.3), $config["safe"] = 1 to auto-configure ceratin htmLawed parameters for maximizing security (section 3.6), etc.

  Permitting the *style* attribute brings in risks of click-jacking, phishing, web-page overlays, etc., even when the safe parameter is enabled (see section 3.6). Except for URLs and a few other things like CSS dynamic expressions, htmLawed currently does not check every CSS style property. It does provide ways for the code-developer implementing htmLawed to do such checks through htmLawed's $spec argument, and through the hook_tag parameter (see section 3.4.8 for more). Disallowing style completely and relying on CSS classes and stylesheet files is recommended.

  htmLawed does not check or correct the character encoding of the input it receives. In conjunction with permissive circumstances, such as when the character encoding is left undefined through HTTP headers or HTML meta tags, this can allow for an exploit (like Google's UTF-7/XSS vulnerability of the past).

  Ocassionally, though very rarely, the default settings with which htmLawed runs may change between different versions of htmLawed. Admins should keep this in mind when upgrading htmLawed. Important changes in htmLawed's default behavior in new releases of the software are noted in section 4.5 on upgrades.

2.6  Use with kses() code

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  The Kses PHP script for HTML filtering is used by many applications (like WordPress, as in year 2012). It is possible to have such applications use htmLawed instead, since it is compatible with code that calls the kses() function declared in the Kses file (usually named kses.php). E.g., application code like this will continue to work after replacing Kses with htmLawed:

    $comment_filtered = kses($comment_input, array('a'=>array(), 'b'=>array(), 'i'=>array()));

  If the application uses a Kses file that has the kses() function declared, then, to have the application use htmLawed instead of Kses, rename htmLawed.php (to kses.php, e.g.) and replace the Kses file (or just replace the code in the Kses file with the htmLawed code). If the kses() function in the Kses file had been renamed by the application developer (e.g., in WordPress, it is named wp_kses()), then appropriately rename the kses() function in the htmLawed code. Then, add the following code (which was a part of htmLawed prior to version 1.2):

    // kses compatibility
    function kses($t, $h, $p=array('http', 'https', 'ftp', 'news', 'nntp', 'telnet', 'gopher', 'mailto')){
     foreach($h as $k=>$v){
      $h[$k]['n']['*'] = 1;
     $C['cdata'] = $C['comment'] = $C['make_tag_strict'] = $C['no_deprecated_attr'] = $C['unique_ids'] = 0;
     $C['keep_bad'] = 1;
     $C['elements'] = count($h) ? strtolower(implode(',', array_keys($h))) : '-*';
     $C['hook'] = 'kses_hook';
     $C['schemes'] = '*:'. implode(',', $p);
     return htmLawed($t, $C, $h);

    function kses_hook($t, &$C, &$S){
     return $t;

  If the Kses file used by the application has been significantly altered by the application developers, then one may need a different approach. E.g., with WordPress (as in the year 2012), it is best to copy the htmLawed code, along with the above-mentioned additions, to wp_includes/kses.php, rename the newly added function kses() to wp_kses(), and delete the code for the original wp_kses() function.

  If the Kses code has a non-empty hook function (e.g., wp_kses_hook() in case of WordPress), then the code for htmLawed's kses_hook() function should be appropriately edited. However, the requirement of the hook function should be re-evaluated considering that htmLawed has extra capabilities. With WordPress, the hook function is an essential one. The following code is suggested for the htmLawed kses_hook() in case of WordPress:

    // kses compatibility
    function kses_hook($string, &$cf, &$spec){
     $allowed_html = $spec;
     $allowed_protocols = array();
     foreach($cf['schemes'] as $v){
      foreach($v as $k2=>$v2){
       if(!in_array($k2, $allowed_protocols)){
        $allowed_protocols[] = $k2;
     return wp_kses_hook($string, $allowed_html, $allowed_protocols);

2.7  Tolerance for ill-written HTML

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  htmLawed can work with ill-written HTML code in the input. However, HTML that is too ill-written may not be read as HTML, and may therefore get identified as mere plain text. Following statements indicate the degree of looseness that htmLawed can work with, and can be provided in instructions to writers:

  *  Tags must be flanked by < and > with no > inside -- any needed > should be put in as &gt;. It is possible for tag content (element name and attributes) to be spread over many lines instead of being on one. A space may be present between the tag content and >, like <div > and <img / >, but not after the <.

  *  Element and attribute names need not be lower-cased.

  *  Attribute string of elements may be liberally spaced with tabs, line-breaks, etc.

  *  Attribute values may be single- and not double-quoted.

  *  Left-padding of numeric entities (like, &#0160;, &x07ff;) with 0 is okay as long as the number of characters between between the & and the ; does not exceed 8. All entities must end with ; though.

  *  Named character entities must be properly cased. Thus, &Lt; or &TILDE; will not be recognized as entities and will be neutralized.

  *  HTML comments should not be inside element tags (they can be between tags), and should begin with <!-- and end with -->. Characters like <, >, and & may be allowed inside depending on $config, but any --> inside should be put in as --&gt;. Any -- inside will be automatically converted to -, and a space will be added before the --> comment-closing marker  unless $config["comments"] is set to 4 (section 3.3.1).

  *  CDATA sections should not be inside element tags, and can be in element content only if plain text is allowed for that element. They should begin with <[CDATA[ and end with ]]>. Characters like <, >, and & may be allowed inside depending on $config, but any ]]> inside should be put in as ]]&gt;.

  *  For attribute values, character entities &lt;, &gt; and &amp; should be used instead of characters < and >, and & (when & is not part of a character entity). This applies even for Javascript code in values of attributes like onclick.

  *  Characters <, >, & and " that are part of actual Javascript, etc., code in script elements should be used as such and not be put in as entities like &gt;. Otherwise, though the HTML will be valid, the code may fail to work. Further, if such characters have to be used, then they should be put inside CDATA sections.

  *  Simple instructions like "an opening tag cannot be present between two closing tags" and "nested elements should be closed in the reverse order of how they were opened" can help authors write balanced HTML. If tags are imbalanced, htmLawed will try to balance them, but in the process, depending on $config["keep_bad"], some code/text may be lost.

  *  Input authors should be notified of admin-specified allowed elements, attributes, configuration values (like conversion of named entities to numeric ones), etc.

  *  With $config["unique_ids"] not 0 and the id attribute being permitted, writers should carefully avoid using duplicate or invalid id values as even though htmLawed will correct/remove the values, the final output may not be the one desired. E.g., when <a id="home"></a><input id="home" /><label for="home"></label> is processed into
<a id="home"></a><input id="prefix_home" /><label for="home"></label>.

  *  Even if intended HTML is lost from an ill-written input, the processed output will be more secure and standard-compliant.

  *  For URLs, unless $config["scheme"] is appropriately set, writers should avoid using escape characters or entities in schemes. E.g., htt&#112; (which many browsers will read as the harmless http) may be considered bad by htmLawed.

  *  htmLawed will attempt to put plain text present directly inside blockquote, form, map and noscript elements (illegal as per the specifications) inside auto-generated div elements during tag balancing (section 3.3.3).

2.8  Limitations & work-arounds

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  htmLawed's main objective is to make the input text more standard-compliant, secure for readers, and free of HTML elements and attributes considered undesirable by the administrator. Some of its current limitations, regardless of this objective, are noted below along with possible work-arounds.

  It should be borne in mind that no browser application is 100% standard-compliant, standard specifications continue to evolve, and many browsers accept commonly used non-standard HTML. Regarding security, note that unsafe HTML code is not legally invalid per se.

  *  htmLawed might not strictly adhere to current HTML standards as standard specification for HTML by WHATWG is continuously evolving, and there is laxity among HTML interpreters (browsers) regarding standards. Admins can configure htmLawed to be more strict about standard compliance.

  *  In general, htmLawed processes input to generate output that is most likely to be standard-compatible in most users' browsers. Thus, for example, it does not enforce the required value of 0 on border attribute of img (an HTML version 5 specification).

  *  htmLawed is meant for input that goes into the body of HTML documents. HTML's head-level elements are not supported, nor are the frame-specific elements frameset, frame and noframes. However, content of the latter elements can be individually filtered through htmLawed.

  *  It cannot handle input that has non-HTML code like SVG and MathML. One way around is to break the input into pieces and passing only those without non-HTML code to htmLawed. Another is described in section 3.9. A third way may be to some how take advantage of the $config["and_mark"] parameter (see section 3.2).

  *  By default, htmLawed won't check many attribute values for standard compliance. E.g., width="20m" with the dimension in non-standard m is let through. Implementing universal and strict attribute value checks can make htmLawed slow and resource-intensive. Admins should look at the hook_tag parameter (section 3.4.9) or $spec to enforce finer checks on attribute values.

  *  By default, htmLawed considers all ARIA, data-*, event, and microdata attributes as global attributes and permits them in all elements. This is not strictly standard-compliant. E.g., the itemtype microdata attribute is permitted only in elements that also have the itemscope attribute. Admins can configure htmLawed to be more strict about this (section 2.3).

  *  The attributes, whether deprecated (which can be transformed by htmLawed) or not, that it supports are largely those that are in the specifications. Only a few of the proprietary attributes are supported. However, $spec can be used to allow custom attributes (section 2.3).

  *  Except for contained URLs and dynamic expressions (also optional), htmLawed does not check CSS style property values. Admins should look at using the hook_tag parameter (section 3.4.9) or $spec for finer checks. Perhaps the best option is to disallow style but allow class attributes with the right oneof or match values for class, and have the various class style properties in .css CSS stylesheet files.

  *  htmLawed does not parse emoticons, decode BBcode, or wikify, auto-converting text to proper HTML. Similarly, it won't convert line-breaks to br elements. Such functions are beyond its purview. Admins should use other code to pre- or post-process the input for such purposes.

  *  htmLawed cannot be used to have links force-opened in new windows (by auto-adding appropriate target and onclick attributes to a). Admins should look at Javascript-based DOM-modifying solutions for this. Admins may also be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  Nesting-based checks are not possible. E.g., one cannot disallow p elements specifically inside td while permitting it elsewhere. Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  Except for optionally converting absolute or relative URLs to the other type, htmLawed will not alter URLs (e.g., to change the value of query strings or to convert http to https. Having absolute URLs may be a standard-requirement, e.g., when HTML is embedded in email messages, whereas altering URLs for other purposes is beyond htmLawed's goals. Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  Pairs of opening and closing tags that do not enclose any content (like <em></em>) are not removed. This may be against the standard specification for certain elements (e.g., table). However, presence of such standard-incompliant code will not break the display or layout of content. Admins can also use simple regex-based code to filter out such code.

  *  htmLawed does not check for certain element orderings described in the standard specifications (e.g., in a table, tbody is allowed before tfoot). Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  htmLawed does not check the number of nested elements. E.g., it will allow two caption elements in a table element, illegal as per standard specifications. Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  There are multiple ways to interpret ill-written HTML. E.g., in <small><small>text</small>, is it that the second closing tag for small is missing or is it that the second opening tag for small was put in by mistake? htmLawed corrects the HTML in the string assuming the former, while the user may have intended the string for the latter. This is an issue that is impossible to address perfectly.

  *  htmLawed might convert certain entities to actual characters and remove backslashes and CSS comment-markers (/*) in style attribute values in order to detect malicious HTML like crafted, Internet Explorer browser-specific dynamic expressions like &#101;xpression.... If this is too harsh, admins can allow CSS expressions through htmLawed core but then use a custom function through the hook_tag parameter (section 3.4.9) to more specifically identify CSS expressions in the style attribute values. Also, using $config["style_pass"], it is possible to have htmLawed pass style attribute values without even looking at them (section 3.4.8).

  *  htmLawed does not correct certain possible attribute-based security vulnerabilities (e.g., <a href="http://x%22+style=%22background-image:xss">x</a>). These arise when browsers mis-identify markup in escaped text, defeating the very purpose of escaping text (a bad browser will read the given example as <a href="http://x" style="background-image:xss">x</a>).

  *  Because of inadequate Unicode support in PHP, htmLawed does not remove the high value HTML-invalid characters with multi-byte code-points. Such characters however are extremely unlikely to be in the input. (see section 3.1).

  *  htmLawed does not check or correct the character encoding of the input it receives. In conjunction with permitting circumstances such as when the character encoding is left undefined through HTTP headers or HTML meta tags, this can permit an exploit (like Google's UTF-7/XSS vulnerability of the past). Also, htmLawed can mangle input text if it is not well-formed in terms of character encoding. Administrators can consider using code available elsewhere to check well-formedness of input text characters to correct any defect.

  *  htmLawed is expected to work with input texts in ASCII standard-compatible single-byte encodings such as national variants of ASCII (like ISO-646-DE/German of the ISO 646 standard), extended ASCII variants (like ISO 8859-10/Turkish of the ISO 8859/ISO Latin standard), ISO 8859-based Windows variants (like Windows 1252), EBCDIC, Shift JIS (Japanese), GB-Roman (Chinese), and KS-Roman (Korean). It should also properly handle texts with variable-byte encodings like UTF-7 (Unicode) and UTF-8 (Unicode). However, htmLawed may mangle input texts with double-byte encodings like UTF-16 (Unicode), JIS X 0208:1997 (Japanese) and K SX 1001:1992 (Korean), or the UTF-32 (Unicode) quadruple-byte encoding. If an input text has such an encoding, administrators can use PHP's iconv functions, or some other mean, to convert text to UTF-8 before passing it to htmLawed.

  *  Like any script using PHP's PCRE regex functions, PHP setup-specific low PCRE limit values can cause htmLawed to at least partially fail with very long input texts.

2.9  Examples of usage

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  Safest, allowing only safe HTML markup --

    $config = array('safe'=>1);
    $out = htmLawed($in, $config);

  Simplest, allowing all valid HTML markup including Javascript --

    $out = htmLawed($in);

  Allowing all valid HTML markup but restricting URL schemes in src attribute values to http and https --

    $config = array('schemes'=>'*:*; src:http, https');
    $out = htmLawed($in, $config);

  Allowing only safe HTML and the elements a, em, and strong --

    $config = array('safe'=>1, 'elements'=>'a, em, strong');
    $out = htmLawed($in, $config);

  Not allowing elements script and object --

    $config = array('elements'=>'* -script -object');
    $out = htmLawed($in, $config);

  Not allowing attributes id and style --

    $config = array('deny_attribute'=>'id, style');
    $out = htmLawed($in, $config);

  Permitting only attributes title and href --

    $config = array('deny_attribute'=>'* -title -href');
    $out = htmLawed($in, $config);

  Remove bad/disallowed tags altogether instead of converting them to entities --

    $config = array('keep_bad'=>0);
    $out = htmLawed($in, $config);

  Allowing attribute title only in a and not allowing attributes id, style, or scriptable on* attributes like onclick --

    $config = array('deny_attribute'=>'title, id, style, on*');
    $spec = 'a=title';
    $out = htmLawed($in, $config, $spec);

  Allowing a custom attribute, vFlag, in img and permitting custom use of the standard attribute, rel, in input --

    $spec = 'img=vFlag; input=rel';
    $out = htmLawed($in, $config, $spec);

  Some case-studies are presented below.

  1. A blog administrator wants to allow only a, em, strike, strong and u in comments, but needs strike and u transformed to span for better XHTML 1-strict compliance, and, he wants the a links to point only to http or https resources:

    $processed = htmLawed($in, array('elements'=>'a, em, strike, strong, u', 'make_tag_strict'=>1, 'safe'=>1, 'schemes'=>'*:http, https'), 'a=href');

  2. An author uses a custom-made web application to load content on his website. He is the only one using that application and the content he generates has all types of HTML, including scripts. The web application uses htmLawed primarily as a tool to correct errors that creep in while writing HTML and to take care of the occasional bad characters in copy-paste text introduced by Microsoft Office. The web application provides a preview before submitted input is added to the content. For the previewing process, htmLawed is set up as follows:

    $processed = htmLawed($in, array('css_expression'=>1, 'keep_bad'=>1, 'make_tag_strict'=>1, 'schemes'=>'*:*', 'valid_xhtml'=>1));

  For the final submission process, keep_bad is set to 6. A value of 1 for the preview process allows the author to note and correct any HTML mistake without losing any of the typed text.

  3. A data-miner is scraping information in a specific table of similar web-pages and is collating the data rows, and uses htmLawed to reduce unnecessary markup and white-spaces:

    $processed = htmLawed($in, array('elements'=>'tr, td', 'tidy'=>-1), 'tr, td =');

3  Details

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3.1  Invalid/dangerous characters

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  Valid characters (more correctly, their code-points) in HTML or XML are, hexadecimally, 9, a, d, 20 to d7ff, and e000 to 10ffff, except fffe and ffff (decimally, 9, 10, 13, 32 to 55295, and 57344 to 1114111, except 65534 and 65535). htmLawed removes the invalid characters 0 to 8, b, c, and e to 1f.

  Because of PHP's poor native support for multi-byte characters, htmLawed cannot check for the remaining invalid code-points. However, for various reasons, it is very unlikely for any of those characters to be in the input.

  Characters that are discouraged (see section 5.1) but not invalid are not removed by htmLawed.

  It (function hl_tag()) also replaces the potentially dangerous (in some Mozilla [Firefox] and Opera browsers) soft-hyphen character (code-point, hexadecimally, ad, or decimally, 173) in attribute values with spaces. Where required, the characters <, >, &, and " are converted to entities.

  With $config["clean_ms_char"] set as 1 or 2, many of the discouraged characters (decimal code-points 127 to 159 except 133) that many Microsoft applications incorrectly use (as per the Windows 1252 [Cp-1252] or a similar encoding system), and the character for decimal code-point 133, are converted to appropriate decimal numerical entities (or removed for a few cases)-- see appendix in section 5.4. This can help avoid some display issues arising from copying-pasting of content.

  With $config["clean_ms_char"] set as 2, characters for the hexadecimal code-points 82, 91, and 92 (for special single-quotes), and 84, 93, and 94 (for special double-quotes) are converted to ordinary single and double quotes respectively and not to entities.

  The character values are replaced with entities/characters and not character values referred to by the entities/characters to keep this task independent of the character-encoding of input text.

  The $config["clean_ms_char"] parameter should not be used if authors do not copy-paste Microsoft-created text, or if the input text is not believed to use the Windows 1252 (Cp-1252) or a similar encoding like Cp-1251 (otherwise, for example when UTF-8 encoding is in use, Japanese or Korean characters can get mangled). Further, the input form and the web-pages displaying it or its content should have the character encoding appropriately marked-up.

3.2  Character references/entities

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  Valid character entities take the form &*; where * is #x followed by a hexadecimal number (hexadecimal numeric entity; like &#xA0; for non-breaking space), or alphanumeric like gt (external or named entity; like &nbsp; for non-breaking space), or # followed by a number (decimal numeric entity; like &#160; for non-breaking space). Character entities referring to the soft-hyphen character (the &shy; or \xad character; hexadecimal code-point ad [decimal 173]) in URL-accepting attribute values are always replaced with spaces; soft-hyphens in attribute values introduce vulnerabilities in some older versions of the Opera and Mozilla [Firefox] browsers.

  htmLawed (function hl_entity()):

  *  Neutralizes entities with multiple leading zeroes or missing semi-colons (potentially dangerous)

  *  Lowercases the X (for XML-compliance) and A-F of hexadecimal numeric entities

  *  Neutralizes entities referring to characters that are HTML-invalid (see section 3.1)

  *  Neutralizes entities referring to characters that are HTML-discouraged (code-points, hexadecimally, 7f to 84, 86 to 9f, and fdd0 to fddf, or decimally, 127 to 132, 134 to 159, and 64991 to 64976). Entities referring to the remaining discouraged characters (see section 5.1 for a full list) are let through.

  *  Neutralizes named entities that are not in the HTML5 specification

  *  Optionally converts valid HTML-specific named entities except &gt;, &lt;, &quot;, and &amp; to decimal numeric ones (hexadecimal if $config["hexdec_entity"] is 2) for generic XML-compliance. For this, $config["named_entity"] should be 1.

  *  Optionally converts hexadecimal numeric entities to the more widely supported decimal ones. For this, $config["hexdec_entity"] should be 0.

  *  Optionally converts decimal numeric entities to the hexadecimal ones. For this, $config["hexdec_entity"] should be 2.

  Neutralization refers to the entitification of & to &amp;.

  Note: htmLawed does not convert entities to the actual characters represented by them; one can pass the htmLawed output through PHP's html_entity_decode function for that.

  Note: If $config["and_mark"] is set, and set to a value other than 0, then the & characters in the original input are replaced with the control character for the hexadecimal code-point 6 (\x06; & characters introduced by htmLawed, e.g., after converting < to &lt;, are not affected). This allows one to distinguish, say, an &gt; introduced by htmLawed and an &gt; put in by the input writer, and can be helpful in further processing of the htmLawed-processed text (e.g., to identify the character sequence o(><)o to generate an emoticon image). When this feature is active, admins should ensure that the htmLawed output is not directly used in web pages or XML documents as the presence of the \x06 can break documents. Before use in such documents, and preferably before any storage, any remaining \x06 should be changed back to &, e.g., with:

    $final = str_replace("\x06", '&', $prelim);

  Also, see section 3.9.

3.3  HTML elements

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  htmLawed can be configured to allow only certain HTML elements (tags) in the input. Disallowed elements (just tag-content, and not element-content), based on $config["keep_bad"], are either neutralized (converted to plain text by entitification of < and >) or removed.

  E.g., with only em permitted:


      <em>My</em> website is <a href="></a>.

  Output, with $config["keep_bad"] = 0:

      <em>My</em> website is

  Output, with $config["keep_bad"] not 0:

      <em>My</em> website is &lt;a href=""&gt;;/a&gt;.

  See section 3.3.3 for differences between the various non-zero $config["keep_bad"] values.

  htmLawed by default permits these 122 HTML elements:

    a, abbr, acronym, address, applet, area, article, aside, audio, b, bdi, bdo, big, blockquote, br, button, canvas, caption, center, cite, code, col, colgroup, command, data, datalist, dd, del, details, dfn, dialog, dir, div, dl, dt, em, embed, fieldset, figcaption, figure, font, footer, form, h1, h2, h3, h4, h5, h6, header, hgroup, hr, i, iframe, img, input, ins, isindex, kbd, keygen, label, legend, li, link, main, map, mark, menu, meta, meter, nav, noscript, object, ol, optgroup, option, output, p, param, picture, pre, progress, q, rb, rbc, rp, rt, rtc, ruby, s, samp, script, section, select, slot, small, source, span, strike, strong, style, sub, summary, sup, table, tbody, td, template, textarea, tfoot, th, thead, time, tr, track, tt, u, ul, var, video, wbr

  htmLawed also supports use of custom HTML elements, but this support can be turned off when $config is appropriately set (i.e., in default configuration, such elements are permitted); see section 3.3.6.

  Elements math and svg are not supported. They and their content will get filtered unless a strategy like in section 3.9 is used.

  Elements like acronym, applet, basefont, bgsound, big, blink, center, command, dir, font, hgroup, image, keygen, marquee, menuitem, nobr, noembed, rb, rtc, shadow, spacer, strike, tt, and xmp are currently obsolete/deprecated. Some of them, like acronym and keygen, are supported in htmLawed (see above list). Tag transformation is possible for improving compliance with HTML standards -- most, but not all, of the obsolete/deprecated elements are converted to valid  ones; see section 3.3.2.

  These 16 htmLawed-supported elements are empty elements that have an opening tag with possible content but no element content (thus, no closing tag): area, br, col, command, embed, hr, img, input, isindex, keygen, link, meta, param, source, track, and wbr.

  As per standards, closing tags are optional for these elements under certain conditions: caption, colgroup, dd, dt, li, optgroup, option, p, rp, rt, tbody, td, tfoot, th, thead, and tr. By default, htmLawed will add a missing closing tag for such elements, unless balancing (section 3.3.3) is turned off.

  With $config["safe"] = 1, the default set of htmLawed-supported elements will exclude applet, audio, canvas, dialog, embed, iframe, object, script and video; see section 3.6.

  When $config["elements"], which specifies allowed elements, is properly defined, and neither empty nor set to 0 or *, the default set is not used. To have elements added to or removed from the default set, a +/- notation is used. E.g., *-script-object implies that only script and object are disallowed, whereas *+noembed means that noembed is also allowed. For an element with a hyphen in name, use round brackets around the name; e.g., (my-custom-element). Elements can also be specified as comma separated names. E.g., a, b, i means only a, b and i are permitted. In this notation, *, + and - have no significance and can actually cause a mis-reading.

  Some more examples of $config["elements"] values indicating permitted elements (note that empty spaces are liberally allowed for clarity):

  *  a, blockquote, code, em, strong -- only a, blockquote, code, em, and strong
  *  *-script -- all excluding script
  *  * -acronym -big -center -dir -font -isindex -s -strike -tt -- only non-obsolete/deprecated elements of HTML5
  *  *+noembed-script -- all including noembed excluding script
  *  *+noembed+(my-custom-element) -- all including noembed and my-custom-element

  Some mis-usages (and the resulting permitted elements) that can be avoided:

  *  -* -- none; instead of htmLawed, one might just use, e.g., the htmlspecialchars() PHP function
  *  *, -script -- all except script; admin probably meant *-script
  *  -*, a, em, strong -- all; admin probably meant a, em, strong
  *  * -- all; admin need not have set elements
  *  *-form+form -- all; a + will always over-ride any -
  *  *, noembed -- only noembed; admin probably meant *+noembed
  *  a, +b, i -- only a and i; admin probably meant a, b, i

  Basically, when using the +/- notation, commas (,) should not be used, and vice versa, and * should be used with the former but not the latter.

  Note: Even if an element that is not in the default set is allowed through $config["elements"], like noembed in the last example, it will eventually be removed during tag balancing unless such balancing is turned off ($config["balance"] set to 0). Currently, the only way around this, which actually is simple, is to edit htmLawed's PHP code which define various arrays in the function hl_balance() to accommodate the element and its nesting properties.

  A possible second way to specify allowed elements is to set $config["parent"] to an element name that supposedly will hold the input, and to set $config["balance"] to 1. During tag balancing (see section 3.3.3), all elements that cannot legally nest inside the parent element will be removed. The parent element is auto-reset to div if $config["parent"] is empty, body, or an element not in htmLawed's default set of 122 elements.

3.3.1  Handling of comments & CDATA sections

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  CDATA sections have the format <![CDATA[...anything but not "]]>"...]]>, and HTML comments, <!--...anything but not "-->"... -->. Neither HTML comments nor CDATA sections can reside inside tags. HTML comments can exist anywhere else, but CDATA sections can exist only where plain text is allowed (e.g., immediately inside td element content but not immediately inside tr element content).

  htmLawed (function hl_commentCdata()) handles HTML comments or CDATA sections depending on the values of $config["comment"] or $config["cdata"]. If 0, such markup is not looked for and the text is processed like plain text. If 1, it is removed completely. If 2, it is preserved but any <, > and & inside are changed to entities. If 3 for $config["cdata"], or 3 or 4 for $config["comment"], they are left as such. When $config["comment"] is set to 4, htmLawed will not force a space character before the --> comment-closing marker. While such a space is required for standard-compliance, it can corrupt marker code put in HTML by some software (such as Microsoft Outlook).

  Note that for the last two cases, HTML comments and CDATA sections will always be removed from tag content (function hl_tag()).


    <!-- home link--><a href="home.htm"><![CDATA[x=&y]]>Home</a>
  Output ($config["comment"] = 0, $config["cdata"] = 2):
    &lt;-- home link--&gt;<a href="home.htm"><![CDATA[x=&amp;y]]>Home</a>
  Output ($config["comment"] = 1, $config["cdata"] = 2):
    <a href="home.htm"><![CDATA[x=&amp;y]]>Home</a>
  Output ($config["comment"] = 2, $config["cdata"] = 2):
    <!-- home link --><a href="home.htm"><![CDATA[x=&amp;y]]>Home</a>
  Output ($config["comment"] = 2, $config["cdata"] = 1):
    <!-- home link --><a href="home.htm">Home</a>
  Output ($config["comment"] = 3, $config["cdata"] = 3):
    <!-- home link --><a href="home.htm"><![CDATA[x=&y]]>Home</a>
  Output ($config["comment"] = 4, $config["cdata"] = 3):
    <!-- home link--><a href="home.htm"><![CDATA[x=&y]]>Home</a>

  For standard-compliance, comments are given the form <!--comment -->, and any -- in the content is made -. When $config["comment"] is set to 4, htmLawed will not force a space character before the --> comment-closing marker.

  When $config["safe"] = 1, CDATA sections and comments are considered plain text unless $config["comment"] or $config["cdata"] is explicitly specified; see section 3.6.

3.3.2  Tag-transformation for better compliance with standards

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  If $config["make_tag_strict"] is set and not 0, following deprecated elements (and attributes), even if admin-permitted, are mutated as indicated (element content remains intact; function hl_deprecatedElement()):

  *  acronym - abbr
  *  applet - based on $config["make_tag_strict"], unchanged (1) or removed (2)
  *  big - span style="font-size: larger;"
  *  center - div style="text-align: center;"
  *  dir - ul
  *  font (face, size, color) - span style="font-family: ; font-size: ; color: ;" (size transformation reference)
  *  isindex - based on $config["make_tag_strict"], unchanged (1) or removed (2)
  *  s - span style="text-decoration: line-through;"
  *  strike - span style="text-decoration: line-through;"
  *  tt - code

  For an element with a pre-existing style attribute value, the extra style properties are appended.

  Example input:

     The PHP <s>software</s> script used for this <strike>web-page</strike> web-page is <font style="font-weight: bold " face=arial size='+3' color   =  "red  ">htmLawedTest.php</font>, from <u style= 'color:green'>PHP Labware</u>.

  The output:

    <div style="text-align: center;">
     The PHP <span style="text-decoration: line-through;">software</span> script used for this <span style="text-decoration: line-through;">web-page</span> web-page is <span style="font-weight: bold; font-size: 200%; color: red; font-family: arial;">htmLawedTest.php</span>, from <u style="color:green">PHP Labware</u>.

3.3.3  Tag balancing & proper nesting

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  If $config["balance"] is set to 1, htmLawed (function hl_balance()) checks and corrects the input to have properly balanced tags and legal element content (i.e., any element nesting should be valid, and plain text may be present only in the content of elements that allow them).

  Depending on the value of $config["keep_bad"] (see section 2.2 and section 3.3), illegal content may be removed or neutralized to plain text by converting < and > to entities:

  0 - remove; this option is available only to maintain Kses-compatibility and should not be used otherwise (see section 2.6)
  1 - neutralize tags and keep element content
  2 - remove tags but keep element content
  3 and 4 - like 1 and 2, but keep element content only if text (pcdata) is valid in parent element as per specs
  5 and 6 -  like 3 and 4, but line-breaks, tabs and spaces are left

  Example input (disallowing the p element):

    <*> Pseudo-tags <*>
    <xml>Non-HTML tag xml</xml>
    Disallowed tag p

  The output with $config["keep_bad"] = 1:

    &lt;*&gt; Pseudo-tags &lt;*&gt;
    &lt;xml&gt;Non-HTML tag xml&lt;/xml&gt;
    Disallowed tag p

  The output with $config["keep_bad"] = 3:

    &lt;*&gt; Pseudo-tags &lt;*&gt;
    &lt;xml&gt;Non-HTML tag xml&lt;/xml&gt;
    Disallowed tag p

  The output with $config["keep_bad"] = 6:

    &lt;*&gt; Pseudo-tags &lt;*&gt;
    Non-HTML tag xml

    Disallowed tag p


  An option like 1 is useful, e.g., when a writer previews his submission, whereas one like 3 is useful before content is finalized and made available to all.

  Note: In the example above, unlike <*>, <xml> gets considered as a tag (even though there is no HTML element named xml). Thus, the keep_bad parameter's value affects <xml> but not <*>. In general, text matching the regular expression pattern <(/?)([a-zA-Z][a-zA-Z1-6]*)([^>]*?)\s?> is considered a tag (phrase enclosed by the angled brackets < and >, and starting [with an optional slash preceding] with an alphanumeric word that starts with an alphabet...), and is subjected to the keep_bad value.

  Nesting/content rules for each of the 122 standard elements in htmLawed's default set (see section 3.3) are defined in function hl_balance(). Any custom element (section 3.3.6) is permitted to be within and to contain any other element.

  Plain text and/or certain elements nested inside blockquote, form, map and noscript need to be in block-level elements. This point is often missed during manual writing of HTML code. htmLawed attempts to address this during balancing. E.g., if the parent container is set as form, the input B:<input type="text" value="b" />C:<input type="text" value="c" /> is converted to <div>B:<input type="text" value="b" />C:<input type="text" value="c" /></div>.

3.3.4  Elements requiring child elements

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  As per HTML specifications, elements such as those below require legal child elements nested inside them:

    blockquote, dir, dl, form, map, menu, noscript, ol, optgroup, rbc, rtc, ruby, select, table, tbody, tfoot, thead, tr, ul

  In some cases, the specifications stipulate the number and/or the ordering of the child elements. A table can have 0 or 1 caption, tbody, tfoot, and thead, but they must be in this order: caption, thead, tfoot, tbody.

  htmLawed currently does not check for conformance to these rules. Note that any non-compliance in this regard will not introduce security vulnerabilities, crash browser applications, or affect the rendering of web-pages.

  With $config["direct_list_nest"] set to 1, htmLawed will allow direct nesting of ol, ul, or menu list within another ol, ul, or menu without requiring the child list to be within an li of the parent list. While this may not be standard-compliant, directly nested lists are rendered properly by almost all browsers. The parameter $config["direct_list_nest"] has no effect if tag balancing (section 3.3.3) is turned off.

3.3.5  Beautify or compact HTML

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  By default, htmLawed will neither beautify HTML code by formatting it with indentations, etc., nor will it make it compact by removing un-needed white-space.(It does always properly white-space tag content.)

  As per the HTML standards, spaces, tabs and line-breaks in web-pages (except those inside pre elements) are all considered equivalent, and referred to as white-spaces. Browser applications are supposed to consider contiguous white-spaces as just a single space, and to disregard white-spaces trailing opening tags or preceding closing tags. This white-space normalization allows the use of text/code beautifully formatted with indentations and line-spacings for readability. Such pretty HTML can, however, increase the size of web-pages, or make the extraction or scraping of plain text cumbersome.

  With the $config parameter tidy, htmLawed can be used to beautify or compact the input text. Input with just plain text and no HTML markup is also subject to this. Besides pre, the script, and textarea elements, CDATA sections, and HTML comments are not subjected to the tidying process.

  Any custom HTML element (section 3.3.6) is treated like an inline element, like strong, during tidying.

  To compact, use $config["tidy"] = -1; single instances or runs of white-spaces are replaced with a single space, and white-spaces trailing and leading open and closing tags, respectively, are removed.

  To beautify, $config["tidy"] is set as 1, or for customized tidying, as a string like 2s2n. The s or t character specifies the use of spaces or tabs for indentation. The first and third characters, any of the digits 0-9, specify the number of spaces or tabs per indentation, and any parental lead spacing (extra indenting of the whole block of input text). The r and n characters are used to specify line-break characters: n for \n (Unix/Mac OS X line-breaks), rn or nr for \r\n (Windows/DOS line-breaks), or r for \r.

  For instance, with $config["tidy"] set as 3s2n, 3 space characters are used per indentation level, the entire block of text (HTML code) gets a lead (left spacing) of 2 space characters, and line-breaks are with \n character.

  The $config["tidy"] value of 1 is equivalent to 2s0n. Other $config["tidy"] values are read loosely: a value of 4 is equivalent to 4s0n; t2, to 1t2n; s, to 2s0n; 2TR, to 2t0r; T1, to 1t1n; nr3, to 3s0nr, and so on. Except in the indentations and line-spacings, runs of white-spaces are replaced with a single space during beautification.

  Input formatting using $config["tidy"] is not recommended when input text has mixed markup (like HTML + PHP).

3.3.6  Custom HTML elements

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  Custom elements are HTML elements whose properties/behaviors are defined by the author, instead of being universal (i.e., defined by the HTML interpreter like a browser). Their names must begin with a lowercased a-z character, contain at least one hyphen (-), and cannot be: annotation-xml, color-profile, font-face, font-face-src, font-face-uri, font-face-format, font-face-name, missing-glyph. A huge variety of characters is permitted in the name.

    0-9 | . | _ | #xB7 | #xC0-#xD6 | #xD8-#xF6 | #xF8-#x37D | #x37F-#x1FFF | #x200C-#x200D | #x203F-#x2040 | #x2070-#x218F | #x2C00-#x2FEF | #x3001-#xD7FF | #xF900-#xFDCF | #xFDF0-#xFFFD | [#x10000-#xEFFFF]

  With $config["any_custom_element"] set to 0, no custom element is permitted, whereas with a value of 1 (default value), any such element is permitted. Regardless of the setting, specific custom elements can be denied or permitted through $config["elements"] (see section 3.3.1).

  Any custom HTML element is treated like an inline element, like strong, during tidying (section 3.3.5). During tag balancing (section 3.3.3), any custom element is permitted to be within and to contain any other element. These laxities are necessitated because, by definition, custom elements are parochial.

  Custom elements are permitted to have attributes of any name consisting of any character except a few such as equal, forward slash, and most control characters (unless denied through $spec) and satisfying any data attribute name requirement.

3.4  Attributes

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  In its default setting, htmLawed will only permit attributes described in the HTML specifications (including deprecated ones). A list of the attributes and the elements they are allowed in is in section 5.2. Using the $spec argument, htmLawed can be forced to permit custom, non-standard attributes as well as custom rules for standard attributes (section 2.3).

  Custom data-* (data-star) attributes, where the first three characters of the value of star (*) after lower-casing do not equal xml, and the value of star does not have a colon (:), equal-to (=), newline, solidus (/), space or tab character, or any upper-case A-Z character are allowed in all elements. ARIA, event and microdata attributes like aria-live, onclick and itemid are also considered global attributes (section 5.2).

  When $config["deny_attribute"] is not set, or set to 0, or empty (""), all attributes are permitted as per standards. Otherwise, $config["deny_attribute"] can be set in two different ways. One way is as a list of comma-separated names of the denied attributes. on* can be used to refer to the group of potentially dangerous, script-accepting event attributes like onchange that have on at the beginning of their names. Similarly, aria* and data* can be used to respectively refer to the set of all ARIA and data-* attributes. The second way to set $config["deny_attribute"] permits the denying of all but a few attributes globally. The notation is * -attribute1 -attribute2 .... Thus, a value of * -title -href implies that except href and title (where allowed as per standards) all other attributes are to be removed. Terms aria* data*, and on* can be used in this notation, and a whitespace character is necessary before the - character.

  With $config["safe"] = 1 (section 3.6), any on* event attribute is disallowed even if $config["deny_attribute"] is set otherwise (such as * -style -on*).

  The attribute restrictions specified with $config["deny_attribute"] apply to all elements. To deny attributes for only specific elements, $spec (see section 2.3) can be used. $spec can also be used to element-specifically permit an attribute otherwise denied through $config["deny_attribute"].

  Finer restrictions on attributes can also be put into effect through $config["hook_tag"] (section 3.4.9).

  Custom elements are permitted to have attributes of any name consisting of any character except a few such as equal, forward slash, and most control characters (unless denied through $spec) and satisfying any data attribute name requirement.

  htmLawed (function hl_tag()) also:

  *  Lower-cases attribute names
  *  Removes duplicate attributes (last one stays)
  *  Gives attributes the form name="value" and single-spaces them, removing unnecessary white-spacing
  *  Provides required attributes (see section 3.4.1)
  *  Optionally lowercases certain standard attribute values (see section 3.4.5)
  *  Double-quotes values and escapes any " inside them
  *  Replaces the possibly dangerous soft-hyphen characters (hexadecimal code-point ad) in the values with spaces
  *  Allows custom function to additionally filter/modify attribute values (see section 3.4.9)

3.4.1  Auto-addition of XHTML-required attributes

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  If indicated attributes for the following elements are found missing, htmLawed (function hl_tag()) will add them (with values same as attribute names unless indicated otherwise below):

  *  area - alt (area)
  *  area, img - src, alt (image)
  *  bdo - dir (ltr)
  *  form - action
  *  label - command
  *  map - name
  *  optgroup - label
  *  param - name
  *  style - scoped
  *  textarea - rows (10), cols (50)

  Additionally, with $config["xml:lang"] set to 1 or 2, if the lang but not the xml:lang attribute is declared, then the latter is added too, with a value copied from that of lang. This is for better standard-compliance. With $config["xml:lang"] set to 2, the lang attribute is removed (XHTML specification).

  Note that the name attribute for map, invalid in XHTML, is also transformed if required -- see section 3.4.6.

3.4.2  Duplicate/invalid id values

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  If $config["unique_ids"] is 1, htmLawed (function hl_tag()) removes id attributes with values that are not standards-compliant (must not have a space character) or duplicate. If $config["unique_ids"] is a word (without a non-word character like space), any duplicate but otherwise valid value will be appropriately prefixed with the word to ensure its uniqueness.

  Even if multiple inputs need to be filtered (through multiple calls to htmLawed), htmLawed ensures uniqueness of id values as it uses a global variable ($GLOBALS["hl_Ids"] array). Further, an admin can restrict the use of certain id values by presetting this variable before htmLawed is called into use. E.g.:

    $GLOBALS['hl_Ids'] = array('top'=>1, 'bottom'=>1, 'myform'=>1); // id values not allowed in input
    $processed = htmLawed($text); // filter input

3.4.3  URL schemes & scripts in attribute values

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  htmLawed edits attributes that take URLs as values if they are found to contain un-permitted schemes. E.g., if the afp scheme is not permitted, then <a href="afp://"> becomes <a href="denied:afp://">, and if Javascript is not permitted <a onclick="javascript:xss();"> becomes <a onclick="denied:javascript:xss();">.

  By default htmLawed permits these schemes in URLs for the href attribute:

    aim, app, feed, file, ftp, gopher, http, https, javascript, irc, mailto, news, nntp, sftp, ssh, tel, telnet, ws, wss

  Also, only data, file, http, https, javascript, ws and wss are permitted in these attributes that accept URLs:

    action, archive, cite, classid, codebase, data, formaction, itemtype, longdesc, model, pluginspage, pluginurl, poster, src, srcset, style, usemap, and event attributes like onclick

  With $config["safe"] = 1 (section 3.6), the above is changed to disallow app, data and javascript.

  Note: URLs in data-* attribute values are not checked, but $spec (section 2.3) or $config["hook_tag"] (section 3.4.9) can be used for this purpose.

  These default sets are used when $config["schemes"] is not set (see section 2.2). To over-ride the defaults, $config["schemes"] is defined as a string of semi-colon-separated sub-strings of type attribute: comma-separated schemes. E.g., href: mailto, http, https; onclick: javascript; src: http, https. For unspecified attributes, data, file, http, https and javascript are permitted. This can be changed by passing schemes for * in $config["schemes"]. E.g., href: mailto, http, https; *: https, https.

  * (asterisk) can be put in the list of schemes to permit all protocols. E.g., style: *; img: http, https results in protocols not being checked in style attribute values. However, in such cases, any relative-to-absolute URL conversion, or vice versa, (section 3.4.4) is not done. When an attribute is explicitly listed in $config["schemes"], then filtering is dictated by the setting for the attribute, with no effect of the setting for asterisk. That is, the set of attributes that asterisk refers to no longer includes the listed attribute.

  Thus, to allow the xmpp scheme, one can set $config["schemes"] as href: mailto, http, https; *: http, https, xmpp, or href: mailto, http, https, xmpp; *: http, https, xmpp, or *: *, and so on. The consequence of each of these example values will be different (e.g., only the last two but not the first will allow xmpp in href)

  As a side-note, one may find style: * useful as URLs in style attributes can be specified in a variety of ways, and the patterns that htmLawed uses to identify URLs may mistakenly identify non-URL text.

  ! can be put in the list of schemes to disallow all protocols as well as local URLs. Thus, with href: http, style: !, <a href="" style="background-image: url(local.jpg);">CNN</a> will become <a href="" style="background-image: url(denied:local.jpg);">CNN</a>

  With $config["safe"] = 1 (section 3.6), all URLs are disallowed in the style attribute values, unless a rule for style is explicitly specified in $config["schemes"] or $config["style_pass"] (section 3.4.8) is set to 1.

3.4.4  Absolute & relative URLs in attribute values

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  htmLawed can make absolute URLs in attributes like href relative ($config["abs_url"] is -1), and vice versa ($config["abs_url"] is 1). URLs in scripts are not considered for this, and so are URLs like #section_6 (fragment), ?name=Tim#show (starting with query string), and ;var=1?name=Tim#show (starting with parameters). Further, this requires that $config["base_url"] be set properly, with the :// and a trailing slash (/), with no query string, etc. E.g., file:///D:/page/,, or http://localhost/demo/ are okay, but file:///D:/page/?help=1, and http://localhost/demo/index.htm are not.

  For making absolute URLs relative, only those URLs that have the $config["base_url"] string at the beginning are converted. E.g., with $config["base_url"] = "", and become a.gif and z/b.gif respectively, while is not changed.

  When making relative URLs absolute, only values for scheme, network location (host-name) and path values in the base URL are inherited. See section 5.5 for more about the URL specification as per RFC 1808.

3.4.5  Lower-cased, standard attribute values

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  Optionally, for standard-compliance, htmLawed (function hl_tag()) lower-cases standard attribute values to give, e.g., input type="password" instead of input type="Password", if $config["lc_std_val"] is 1. Attribute values matching those listed below for any of the elements listed further below (plus those for the type attribute of button or input) are lower-cased:

    all, auto, baseline, bottom, button, captions, center, chapters, char, checkbox, circle, col, colgroup, color, cols, data, date, datetime, datetime-local, default, descriptions, email, file, get, groups, hidden, image, justify, left, ltr, metadata, middle, month, none, number, object, password, poly, post, preserve, radio, range, rect, ref, reset, right, row, rowgroup, rows, rtl, search, submit, subtitles, tel, text, time, top, url, week

    a, area, bdo, button, col, fieldset, form, img, input, object, ol, optgroup, option, param, script, select, table, td, textarea, tfoot, th, thead, tr, track, xml:space

  The following empty (minimized) attributes are always assigned lower-cased values (same as the attribute names):

    checkbox, checked, command, compact, declare, defer, default, disabled, hidden, inert, ismap, itemscope, multiple, nohref, noresize, noshade, nowrap, open, radio, readonly, required, reversed, selected

3.4.6  Transformation of deprecated attributes

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  If $config["no_deprecated_attr"] is 0, then deprecated attributes are removed and, in most cases, their values are transformed to CSS style properties and added to the style attributes (function hl_tag()). Except for bordercolor for table, tr and td, the scores of proprietary attributes that were never part of any cross-browser standard are not supported in this functionality.

  *  align in caption, div, h, h2, h3, h4, h5, h6, hr, img, input, legend, object, p, table - for img with value of left or right, becomes, e.g., float: left; for div and table with value center, becomes margin: auto; all others become, e.g., text-align: right
  *  bgcolor in table, tbody, td, tfoot, th, thead, tr - E.g., bgcolor="#ffffff" becomes background-color: #ffffff
  *  border in object - E.g., height="10" becomes height: 10px
  *  bordercolor in table, td and tr - E.g., bordercolor=#999999 becomes border-color: #999999;
  *  compact in dl, ol and ul - font-size: 85%
  *  cellspacing in table - cellspacing="10" becomes border-spacing: 10px
  *  clear in br - E.g., 'clear="all" becomes clear: both
  *  height in td and th - E.g., height= "10" becomes height: 10px and height="*" becomes height: auto
  *  hspace in img and object - E.g., hspace="10" becomes margin-left: 10px; margin-right: 10px
  *  language in script - language="VBScript" becomes type="text/vbscript"
  *  name in a, form, iframe, img and map - E.g., name="xx" becomes id="xx"
  *  noshade in hr - border-style: none; border: 0; background-color: gray; color: gray
  *  nowrap in td and th - white-space: nowrap
  *  size in hr - E.g., size="10" becomes height: 10px
  *  vspace in img and object - E.g., vspace="10" becomes margin-top: 10px; margin-bottom: 10px
  *  width in hr, pre, table, td and th - like height

  Example input:

    <img src="j.gif" alt="image" name="dad's" /><img src="k.gif" alt="image" id="dad_off" name="dad" />
    <br clear="left" />
    <hr noshade size="1" />
    <img name="img" src="i.gif" align="left" alt="image" hspace="10" vspace="10" width="10em" height="20" border="1" style="padding:5px;" />
    <table width="50em" align="center" bgcolor="red">
      <td width="20%">
       <div align="center">
        <h3 align="right">Section</h3>
        <p align="right">Para</p>
      <td width="*">
    <br clear="all" />

  And the output with $config["no_deprecated_attr"] = 1:

    <img src="j.gif" alt="image" id="dad's" /><img src="k.gif" alt="image" id="dad_off" />
    <br style="clear: left;" />
    <hr style="border-style: none; border: 0; background-color: gray; color: gray; size: 1px;" />
    <img src="i.gif" alt="image" width="10em" height="20" style="padding:5px; float: left; margin-left: 10px; margin-right: 10px; margin-top: 10px; margin-bottom: 10px; border: 1px;" id="img" />
    <table width="50em" style="margin: auto; background-color: red;">
      <td style="width: 20%;">
       <div style="margin: auto;">
        <h3 style="text-align: right;">Section</h3>
        <p style="text-align: right;">Para</p>
      <td style="width: auto;">
    <br style="clear: both;" />

  For lang, deprecated in XHTML 1.1, transformation is taken care of through $config["xml:lang"]; see section 3.4.1.

  The attribute name is deprecated in form, iframe, and img, and is replaced with id if an id attribute doesn't exist and if the name value is appropriate for id (i.e., doesn't have a non-word character like space). For such replacements for a and map, for which the name attribute is deprecated in XHTML 1.1, $config["no_deprecated_attr"] should be set to 2 (when set to 1, for these two elements, the name attribute is retained).

3.4.7  Anti-spam & href

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  htmLawed (function hl_tag()) can check the href attribute values (link addresses) as an anti-spam (email or link spam) measure.

  If $config["anti_mail_spam"] is not 0, the @ of email addresses in href values like is replaced with text specified by $config["anti_mail_spam"]. The text should be of a form that makes it clear to others that the address needs to be edited before a mail is sent; e.g., <remove_this_antispam>@ (makes the example address a<remove_this_antispam>

  For regular links, one can choose to have a rel attribute with nofollow in its value (which tells some search engines to not follow a link). This can discourage link spammers. Additionally, or as an alternative, one can choose to empty the href value altogether (disable the link).

  For use of these options, $config["anti_link_spam"] should be set as an array with values regex1 and regex2, both or one of which can be empty (like array("", "regex2")) to indicate that that option is not to be used. Otherwise, regex1 or regex2 should be PHP- and PCRE-compatible regular expression patterns: href values will be matched against them and those matching the pattern will accordingly be treated.

  Note that the regular expressions should have delimiters, and be well-formed and preferably fast. Absolute efficiency/accuracy is often not needed.

  An example, to have a rel attribute with nofollow for all links, and to disable links that do not point to domains and

    $config["anti_link_spam"] = array('`.`', '`://\W*(?!(abc\.com|xyz\.org))`');

3.4.8  Inline style properties

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  htmLawed can check URL schemes and dynamic expressions (to guard against Javascript, etc., script-based insecurities) in inline CSS style property values in the style attributes. (CSS properties like background-image that accept URLs in their values are noted in section 5.3.) Dynamic CSS expressions that allow scripting in the IE browser, and can be a vulnerability, can be removed from property values by setting $config["css_expression"] to 1 (default setting). Note that when $config["css_expression"] is set to 1, htmLawed will remove /* from the style values.

  Note: Because of the various ways of representing characters in attribute values (URL-escapement, entitification, etc.), htmLawed might alter the values of the style attribute values, and may even falsely identify dynamic CSS expressions and URL schemes in them. If this is an important issue, checking of URLs and dynamic expressions can be turned off ($config["schemes"] = "*...", see section 3.4.3, and $config["css_expression"] = 0). Alternately, admins can use their own custom function for finer handling of style values through the hook_tag parameter (see section 3.4.9).

  It is also possible to have htmLawed let through any style value by setting $config["style_pass"] to 1.

  As such, it is better to set up a CSS file with class declarations, disallow the style attribute, set a $spec rule (see section 2.3) for class for the oneof or match parameter, and ask writers to make use of the class attribute.

  With $config["safe"] = 1 (section 3.6) , all URLs are disallowed in the style attribute values, unless a rule for style is explicitly specified in $config["schemes"] (section 3.4.3) or $config["style_pass"] is set to 1.

3.4.9  Hook function for tag content

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  It is possible to utilize a custom hook function to alter the tag content htmLawed has finalized (i.e., after it has checked/corrected for required attributes, transformed attributes, lower-cased attribute names, etc.). The function should have two arguments, the first receiving an element name and the second receiving either 0 (in case of a closing tag) or an array of attribute name-value pairs (opening tag). It should return a string  with full HTM markup, either an opening or a closing tag with element name and any string of attributes.

  When $config parameter hook_tag is set to the name of a function or class method, htmLawed (function hl_tag()) will pass on the element name, and the finalized attribute name-value pairs as array elements to the function. The function, after completing a task such as filtering or tag transformation, will typically return an empty string, the full opening tag string like <element_name attribute_1_name="attribute_1_value"...> (for empty elements like img and input, the element-closing slash / should also be included), etc.

  This is a powerful functionality that can be exploited for various objectives: consolidate-and-convert inline style attributes to class, convert embed elements to object, permit only one caption element in a table element, disallow embedding of certain types of media, inject HTML, use CSSTidy to sanitize style attribute values, etc.

  As an example, the custom hook code below can be used to force a series of specifically ordered id attributes on all elements, and a specific param element inside all object elements:

    function my_tag_function($element, $attribute_array=0){

      // If second argument is not received, it means a closing tag is being handled
        return "</$element>";

      static $id = 0;
      // Remove any duplicate element
      if($element == 'param' && isset($attribute_array['allowscriptaccess'])){
        return '';

      $new_element = '';

      // Force a serialized ID number
      $attribute_array['id'] = 'my_'. $id;

      // Inject param for allowscriptaccess
      if($element == 'object'){
        $new_element = '<param id="my_'. $id. '"; allowscriptaccess="never" />';

      $string = '';
      foreach($attribute_array as $k=>$v){
        $string .= " {$k}=\"{$v}\"";

      static $empty_elements = array('area'=>1, 'br'=>1, 'col'=>1, 'command'=>1, 'embed'=>1, 'hr'=>1, 'img'=>1, 'input'=>1, 'isindex'=>1, 'keygen'=>1, 'link'=>1, 'meta'=>1, 'param'=>1, 'source'=>1, 'track'=>1, 'wbr'=>1);

      return "<{$element}{$string}". (array_key_exists($element, $empty_elements) ? ' /' : ''). '>'. $new_element;

  The hook_tag parameter is different from the hook parameter (section 3.7).

  Snippets of hook function code developed by others may be available on the htmLawed website.

3.5  Simple configuration directive for most valid XHTML

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  If $config["valid_xhtml"] is set to 1, some relevant $config parameters (indicated by ~ in section 2.2) are auto-adjusted. This allows one to pass the $config argument with a simpler value. If a value for a parameter auto-set through valid_xhtml is still manually provided, then that value will over-ride the auto-set value.

3.6  Simple configuration directive for most safe HTML

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  Safe HTML refers to HTML that is restricted to reduce the vulnerability for scripting attacks (such as XSS) based on HTML code which otherwise may still be legal and compliant with the HTML standard specifications. When elements such as script and object, and attributes such as onmouseover and style are allowed in the input text, an input writer can introduce malevolent HTML code. Note that what is considered safe depends on the nature of the web application and the trust-level accorded to its users.

  htmLawed allows an admin to use $config["safe"] to auto-adjust multiple $config parameters (such as elements which declares the allowed element-set), which otherwise would have to be manually set. The relevant parameters are indicated by " in section 2.2). Thus, one can pass the $config argument with a simpler value. Having the safe parameter set to 1 is equivalent to setting the following $config parameters to the noted values :

    cdata - 0
    comment - 0
    deny_attribute - on*
    elements - * -applet -audio -canvas -dialog -embed -iframe -object -script -video
    schemes - href: aim, feed, file, ftp, gopher, http, https, irc, mailto, news, nntp, sftp, ssh, tel, telnet, ws, wss; style: !; *:file, http, https, ws, wss

  With safe set to 1, htmLawed considers CDATA sections and HTML comments as plain text, and prohibits the applet, audio, canvas, dialog, embed, iframe, object, script and video elements, and the on* attributes like onclick. ( There are $config parameters like css_expression that are not affected by the value set for safe but whose default values still contribute towards a more safe output.) Further, unless overridden by the value for parameter schemes (see section 3.4.3), the schemes app, data and javascript are not permitted, and URLs with schemes are neutralized so that, e.g., style="moz-binding:url(http://danger)" becomes style="moz-binding:url(denied:http://danger)".

  Admins, however, may still want to completely deny the style attribute, e.g., with code like

    $processed = htmLawed($text, array('safe'=>1, 'deny_attribute'=>'style'));

  With $config["safe"] = 1, all URLs are disallowed in the style attribute values, unless a rule for style is explicitly specified in $config["schemes"] (section 3.4.3) or $config["style_pass"] (section 3.4.8) is set to 1.

  Permitting the style attribute brings in risks of click-jacking, etc. CSS property values can render a page non-functional or be used to deface it. Except for URLs, dynamic expressions, and some other things, htmLawed does not completely check style values. It does provide ways for the code-developer implementing htmLawed to do such checks through the $spec argument, and through the hook_tag parameter (see section 3.4.8 for more). Disallowing style completely and relying on CSS classes and stylesheet files is recommended.

  If a value for a parameter auto-set through safe is still manually provided, then that value can over-ride the auto-set value. E.g., with $config["safe"] = 1 and $config["elements"] = "* +script", script, but not applet, is allowed. Such over-ride does not occur for deny_attribute (for legacy reason) when comma-separated attribute names are provided as the value for this parameter (section 3.4); instead htmLawed will add on* to the value provided for deny_attribute.

  A page illustrating the efficacy of htmLawed's anti-XSS abilities with safe set to 1 against XSS vectors listed by RSnake may be available here.

3.7  Using a hook function

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  If $config["hook"] is not set to 0, then htmLawed will allow preliminarily processed input to be altered by a function or class method named by $config["hook"] before starting the main work (but after handling of characters, entities, HTML comments and CDATA sections -- see code for function htmLawed()). The function should have three arguments – the processed input string, and the finalized $config and $spec arrays, in order – and it should return the string after any manipulation.

  The hook function also allows one to alter the finalized values of $config and $spec.

  Note that the hook parameter is different from the hook_tag parameter (section 3.4.9).

  Snippets of hook function code developed by others may be available on the htmLawed website.

3.8  Obtaining finalized parameter values

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  htmLawed can assign the finalized $config and $spec values to a variable named by $config["show_setting"]. The variable, made global by htmLawed, is set as an array with four keys: config, with the $config value, spec, with the $spec value, time, with a value that is the Unix time (the output of PHP's microtime function) when htmLawed completed filtering, and version, with htmLawed version. Admins should use a PHP-compliant variable name (e.g., one that does not begin with a numerical digit) that does not conflict with variable names in their non-htmLawed code.

  The values, which are also post-hook function (if any), can be used to auto-generate information (on, e.g., the elements that are permitted) for input writers.

3.9  Retaining non-HTML tags in input with mixed markup

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  htmLawed does not remove certain characters that, though invalid, are nevertheless discouraged in HTML documents as per the specifications (see section 5.1). This can be utilized to deal with input that contains mixed markup. Input that may have HTML markup as well as some other markup that is based on the <, > and & characters is considered to have mixed markup. The non-HTML markup can be rather proprietary (like markup for emoticons/smileys), or standard (like MathML or SVG). Or it can be programming code meant for execution/evaluation (such as embedded PHP code).

  To deal with such mixed markup, the input text can be pre-processed to hide the non-HTML markup by specifically replacing the <, > and & characters with some of the HTML-discouraged characters (see section 3.1.2). Post-htmLawed processing, the replacements are reverted.

  An example (mixed HTML and PHP code in input text):

    $text = preg_replace('`<\?php(.+?)\?>`sm', "\x83?php\\1?\x84", $text);
    $processed = htmLawed($text);
    $processed = preg_replace('`\x83\?php(.+?)\?\x84`sm', '<?php$1?>', $processed);

  This code will not work if $config["clean_ms_char"] is set to 1 (section 3.1), in which case one should instead deploy a hook function (section 3.7). (htmLawed internally uses certain control characters, code-points 1 to 7, and use of these characters as markers in the logic of hook functions may cause issues.)

  Admins may also be able to use $config["and_mark"] to deal with such mixed markup; see section 3.2.

4  Other

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4.1  Support

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  Software updates and forum-based community-support may be found at

4.2  Known issues

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  See section 2.8.

4.3  Change-log

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  (The release date for the downloadable package of files containing documentation, demo script, test-cases, etc., besides the htmLawed.php file, may be updated without a change-log entry if the secondary files, but not htmLawed per se, are revised.)

  Version number - Release date. Notes

  1.2.15 - 4 August 2023. Proper checking of attribute formaction; transformation for deprecated attribute bgcolor for tbody, tfoot, and thead; support for URL schemes ws and wss

  1.2.14 - 25 May 2023. Fixed issue that prevented use of attribute srcset in link and source

  1.2.13 - 1 May 2023. Fixed issues with nesting for details and ruby, handling of self-closing tags, handling of multiple values in sizes, and $config["schemes"] parsing

  1.2.12 - 25 April 2023. Fixed issue that prevented use of attribute sizes in img and source

  1.2.11 - 23 January 2023. Fixes an XSS vulnerability arising from a lack of inspection for the alphabetical HTML entity for colon character in URLs

  1.2.10 - 5 November 2022. Class methods can now be specified as $config hook and hook_tag functions; corrects a PHP notice if $config["schemes"] mistakenly lacks colons.

  1.2.9 - 2 July 2022. Improves parsing of $config["deny_attribute"] to permit spaces flanking comma characters and allow references to sets of all ARIA, data-* and event attributes; fixes parsing of $spec for data-* attribute rules; now permits use of aria*, data*, and on* in $spec; now covers all named HTML entities of current standard specification (this increased htmLawed code size by ~40%); recognizes that closing tag may be omitted for caption, optgroup, rp, rt, and tbody as well; recognizes that archive and poster attribute values can have URLs, which can be multiple; recognizes onloadend as global attribute; renames some internal functions; improved standards-compliance for element nesting.

  1.2.8 - 6 June 2022. Fixes incorrect formatting of HTML comments when $config["comment"] = 4; fixes misreading of entity-fied colon characters in style attribute values; $config["show_setting"] now includes htmLawed version; improved PHP 8.2 code compatibility, and readability

  1.2.7 - 10 April 2022. Support for elements dialog, picture, slot, and template; support for custom HTML elements; support for global attributes autocapitalize, autofocus, enterkeyhint, inputmode, is, and nonce; support for 17 additional ARIA and 11 additional on* event handler attributes; support for attributes with names not beginning with a-z; fix for a minor bug arising during deprecated height/weight attribute transformation

  1.2.6 - 4 September 2021. Fixes a bug that arises when $config["deny_attribute"] has a data-* attribute with > 1 hyphen character

  1.2.5 - 24 September 2019. Fixes two bugs in font tag transformation - 16 May 2019. Corrects a PHP notice if a semi-colon is present in $config["schemes"] - 12 September 2017. Corrects a function re-declaration bug introduced in version 1.2.4

  1.2.4 - 31 August 2017. Removes use of PHP create_function function and $php_errormsg reserved variable (deprecated in PHP 7.2)

  1.2.3 - 5 July 2017. New option value of 4 for $config["comments"] to stop enforcing a space character before the --> comment-closing marker

  1.2.2 - 25 May 2017. Fix for a bug in parsing $spec that got introduced in version 1.2; also, $spec is now parsed to accommodate specifications for an HTML element when they are specified in multiple rules - 17 May 2017. Fix for a potential security vulnerability in transformation of deprecated attributes

  1.2.1 - 15 May 2017. Fix for a potential security vulnerability in transformation of deprecated attributes

  1.2 - 11 February 2017. (First beta release on 26 May 2013). Added support for HTML version 5; ARIA, data-* and microdata attributes; app, data, javascript and tel URL schemes (thus, javascript: is not filtered in default mode). Removed support for code using Kses functions (see section 2.6). Changes in revisions to the beta releases are not noted here.

  1.1.22 - 5 March 2016. Improved testing of attribute value rules specified in $spec

  1.1.21 - 27 February 2016. Improvement and security fix in transforming font element

  1.1.20 - 9 June 2015. Fix for a potential security vulnerability arising from unescaped double-quote character in single-quoted attribute value of some deprecated elements when tag transformation is enabled; recognition for non-(HTML 4) standard allowfullscreen attribute of iframe

  1.1.19 - 19 January 2015. Fix for a bug in cleaning of soft-hyphens in URL values, etc

  1.1.18 - 2 August 2014. Fix for a potential security vulnerability arising from specially encoded text with serial opening tags

  1.1.17 - 11 March 2014. Removed use of PHP function preg_replace with e modifier for compatibility with PHP 5.5.

  1.1.16 - 29 August 2013. Fix for a potential security vulnerability arising from specialy encoded space characters in URL schemes/protocols

  1.1.15 - 11 August 2013. Improved tidying/prettifying functionality

  1.1.14 - 8 August 2012. Fix for possible segmental loss of incremental indentation during tidying when balance is disabled; fix for non-effectuation under some circumstances of a corrective behavior to preserve plain text within elements like blockquote

  1.1.13 - 22 July 2012. Added feature allowing use of custom, non-standard attributes or custom rules for standard attributes

  1.1.12 - 5 July 2012. Fix for a bug in identifying an unquoted value of the face attribute

  1.1.11 - 5 June 2012. Fix for possible problem with handling of multi-byte characters in attribute values in an mbstring.func_overload enviroment. $config["hook_tag"], if specified, now receives names of elements in closing tags.

  1.1.10 - 22 October 2011. Fix for a bug in the tidy functionality that caused the entire input to be replaced with a single space; new parameter, $config["direct_list_nest"] to allow direct descendance of a list in a list. (5 April 2012. Dual licensing from LGPLv3 to LGPLv3 and GPLv2+.) - 6 July 2011. Minor correction of a rule for nesting of li within dir - 3 July 2010. Parameter schemes now accepts ! so any URL, even a local one, can be denied. An issue in which a second URL value in style properties was not checked was fixed. - 17 May 2010. Checks for correct nesting of param - 26 April 2010. Minor fix regarding rendering of denied URL schemes - 26 February 2010. htmLawed now uses the LGPL version 3 license; support for flashvars attribute for embed

  1.1.9 - 22 December 2009. Soft-hyphens are now removed only from URL-accepting attribute values - 16 July 2009. Minor code-change to fix a PHP error notice

  1.1.8 - 23 April 2009. Parameter deny_attribute now accepts the wild-card *, making it simpler to specify its value when all but a few attributes are being denied; fixed a bug in interpreting $spec

  1.1.7 - 11-12 March 2009. Attributes globally denied through deny_attribute can be allowed element-specifically through $spec; $config["style_pass"] allowing letting through any style value introduced; altered logic to catch certain types of dynamic crafted CSS expressions

  1.1.3-6 - 28-31 January - 4 February 2009. Altered logic to catch certain types of dynamic crafted CSS expressions

  1.1.2 - 22 January 2009. Fixed bug in parsing of font attributes during tag transformation

  1.1.1 - 27 September 2008. Better nesting correction when omitable closing tags are absent

  1.1 - 29 June 2008. $config["hook_tag"] and $config["tidy"] introduced for custom tag/attribute check/modification/injection and output compaction/beautification; fixed a regex-in-$spec parsing bug

  1.0.9 - 11 June 2008. Fix for a bug in checks for invalid HTML code-point entities

  1.0.8 - 15 May 2008. Permit bordercolor attribute for table, td and tr

  1.0.7 - 1 May 2008. Support for wmode attribute for embed; $config["show_setting"] introduced; improved $config["elements"] evaluation

  1.0.6 - 20 April 2008. $config["and_mark"] introduced

  1.0.5 - 12 March 2008. style URL schemes essentially disallowed when $config safe is on; improved regex for CSS expression search

  1.0.4 - 10 March 2008. Improved corrections for blockquote, form, map and noscript

  1.0.3 - 3 March 2008. Character entities for soft-hyphens are now replaced with spaces (instead of being removed); fix for a bug allowing td directly inside table; $config["safe"] introduced

  1.0.2 - 13 February 2008. Improved implementation of $config["keep_bad"]

  1.0.1 - 7 November 2007. Improved regex for identifying URLs, protocols and dynamic expressions; no error display during regex testing

  1.0 - 2 November 2007. First release

4.4  Testing

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  To test htmLawed using a form interface, a demo web-page is provided with the htmLawed distribution (htmLawed.php and htmLawedTest.php should be in the same directory on the web-server). A file with test-cases is also provided.

4.5  Upgrade, & old versions

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  Upgrading is as simple as replacing the previous version of htmLawed.php, assuming the file was not modified for customized features. As htmLawed output is almost always used in static documents, upgrading should not affect old, finalized content.

  Note: The following upgrades may affect the functionality of a specific htmLawed installation:

  (1) From version 1.1-1.1.10 to 1.1.11 or later, if a hook_tag function is in use: In version 1.1.11 and later, elements in closing tags (and not just the opening tags) are also passed to the function. There are no attribute names/values to pass, so a hook_tag function receives only the element name. The hook_tag function therefore may have to be edited. See section 3.4.9.

  (2) From version older than 1.2.beta to later, if htmLawed was used as Kses replacement with Kses code in use: In version 1.2.beta or later, htmLawed no longer provides direct support for code that uses Kses functions (see section 2.6).

  (3) From version older than 1.2 to later, if htmLawed is used without $config["safe"] set to 1: Unlike previous versions, htmLawed version 1.2 and later permit data and javascript URL schemes by default (see section 3.4.3).

  Old versions of htmLawed may be available online. E.g., for version 1.0, check; for 1.1.1,; and for 1.1.22,

4.6  Comparison with HTMLPurifier

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  The HTMLPurifier PHP library by Edward Yang is a good HTML filtering script that uses object-oriented PHP code. Compared to htmLawed, as of year 2015, HTMLPurifier:

  *  does not support PHP versions older than 5.0 (HTMLPurifier dropped PHP 4 support after version 2)

  *  is 15-20 times bigger (scores of files totalling more than 750 kb)

  *  consumes 10-15 times more RAM memory (just including the HTMLPurifier files without calling the filter requires a few MBs of memory)

  *  is expectedly considerably slower

  *  lacks many of the extra features of htmLawed (like entity conversions and code compaction/beautification)

  *  has poor documentation

  *  may have finer checks for character encodings and attribute values

  *  can log warnings and errors

4.7  Use through application plug-ins/modules

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  Plug-ins/modules to implement htmLawed in applications such as Drupal may have been developed. Check the application websites and the htmLawed forum.

4.8  Use in non-PHP applications

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  Non-PHP applications written in Python, Ruby, etc., may be able to use htmLawed through system calls to the PHP engine. Such code may have been documented on the internet. Also check the forum on the htmLawed site.

4.9  Donate

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  A donation in any currency and amount to appreciate or support this software can be sent by PayPal to this email address: drpatnaik at yahoo dot com.

4.10  Acknowledgements

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  Nicholas Alipaz, Bryan Blakey, Pádraic Brady, Michael Butler, Dac Chartrand, Alexandre Chouinard, NinCollin, Alexandra Ellwood, Ulf Harnhammer, Gareth Heyes, Hakre, Klaus Leithoff, Hideki Mitsuda, jtojnar, Lukasz Pilorz, Shelley Powers, Psych0tr1a, Lincoln Russell, Tomas Sykorka, Harro Verton, walrusmoose, Edward Yang, and many others.

  Thank you!

5  Appendices

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5.1  Characters discouraged in XHTML

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  Characters represented by the following hexadecimal code-points are not invalid, even though some validators may issue messages stating otherwise.

  7f to 84, 86 to 9f, fdd0 to fddf, 1fffe, 1ffff, 2fffe, 2ffff, 3fffe, 3ffff, 4fffe, 4ffff, 5fffe, 5ffff, 6fffe, 6ffff, 7fffe, 7ffff, 8fffe, 8ffff, 9fffe, 9ffff, afffe, affff, bfffe, bffff, cfffe, cffff, dfffe, dffff, efffe, effff, ffffe, fffff, 10fffe and 10ffff

5.2  Valid attribute-element combinations

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  *  includes deprecated attributes (marked ^), attributes for microdata (marked *), some non-standard attributes for embed (marked **), and the non-standard bordercolor; can have multiple comma-separated values (marked %); can have multiple space-separated values (marked $)
  *  only non-frameset, HTML body elements
  *  name for a and map, and lang are invalid in XHTML 1.1
  *  xml:space is only for XHTML 1.1
  *  excludes data-* and author-specified, non-standard attributes of custom elements

  abbr - td, th
  accept - form, input
  accept-charset - form
  action - form
  align - applet, caption^, col, colgroup, div^, embed, h1^, h2^, h3^, h4^, h5^, h6^, hr^, iframe, img^, input^, legend^, object^, p^, table^, tbody, td, tfoot, th, thead, tr
  allowfullscreen - iframe
  alt - applet, area, img, input
  archive - applet, object
  async - script
  autocomplete - input
  autofocus - button, input, keygen, select, textarea
  autoplay - audio, video
  axis - td, th
  bgcolor - embed, table^, tbody^, td^, tfoot^, th^, thead^, tr^
  border - img, object^, table
  bordercolor - table, td, tr
  cellpadding - table
  cellspacing - table
  challenge - keygen
  char - col, colgroup, tbody, td, tfoot, th, thead, tr
  charoff - col, colgroup, tbody, td, tfoot, th, thead, tr
  charset - a, script
  checked - command, input
  cite - blockquote, del, ins, q
  classid - object
  clear - br^
  code - applet
  codebase - object, applet
  codetype - object
  color - font
  cols - textarea
  colspan - td, th
  compact - dir, dl^, menu, ol^, ul^
  content - meta
  controls - audio, video
  coords - area, a
  crossorigin - img
  data - object
  datetime - del, ins, time
  declare - object
  default - track
  defer - script
  dir - bdo
  dirname - input, textarea
  disabled - button, command, fieldset, input, keygen, optgroup, option, select, textarea
  download - a
  enctype - form
  face - font
  flashvars** - embed
  for - label, output
  form - button, fieldset, input, keygen, label, object, output, select, textarea
  formaction - button, input
  formenctype - button, input
  formmethod - button, input
  formnovalidate - button, input
  formtarget - button, input
  frame - table
  frameborder - iframe
  headers - td, th
  height - applet, canvas, embed, iframe, img, input, object, td^, th^, video
  high - meter
  href - a, area, link
  hreflang - a, area, link
  hspace - applet, embed, img^, object^
  icon - command
  ismap - img, input
  keytype - keygen
  keyparams - keygen
  kind - track
  label - command, menu, option, optgroup, track
  language - script^
  list - input
  longdesc - img, iframe
  loop - audio, video
  low - meter
  marginheight - iframe
  marginwidth - iframe
  max - input, meter, progress
  maxlength - input, textarea
  media - a, area, link, source, style
  mediagroup - audio, video
  method - form
  min - input, meter
  model** - embed
  multiple - input, select
  muted - audio, video
  name - a^, applet^, button, embed, fieldset, form^, iframe^, img^, input, keygen, map^, object, output, param, select, slot, textarea
  nohref - area
  noshade - hr^
  novalidate - form
  nowrap - td^, th^
  object - applet
  open - details, dialog
  optimum - meter
  pattern - input
  ping - a, area
  placeholder - input, textarea
  pluginspage** - embed
  pluginurl** - embed
  poster - video
  pqg - keygen
  preload - audio, video
  prompt - isindex
  pubdate - time
  radiogroup* - command
  readonly - input, textarea
  required - input, select, textarea
  rel$ - a, area, link
  rev - a
  reversed - old
  rows - textarea
  rowspan - td, th
  rules - table
  sandbox - iframe
  scope - td, th
  scoped - style
  scrolling - iframe
  seamless - iframe
  selected - option
  shape - area, a
  size - font, hr^, input, select
  sizes - img, link, source
  span - col, colgroup
  src - audio, embed, iframe, img, input, script, source, track, video
  srcdoc~ - iframe
  srclang~ - track
  srcset~% - img, link, source
  standby - object
  start - ol
  step~ - input
  summary - table
  target - a, area, form
  type - a, area, button, command, embed, input, li, link, menu, object, ol, param, script, source, style, ul
  typemustmatch~ - object
  usemap - img, input, object
  valign - col, colgroup, tbody, td, tfoot, th, thead, tr
  value - button, data, input, li, meter, option, param, progress
  valuetype - param
  vspace - applet, embed, img^, object^
  width - applet, canvas, col, colgroup, embed, hr^, iframe, img, input, object, pre^, table, td^, th^, video
  wmode - embed
  wrap~ - textarea

  The following attributes, including event-specific ones and attributes of ARIA and microdata specifications, are considered global and allowed in all elements:

  accesskey, autocapitalize, autofocus, aria-activedescendant, aria-atomic, aria-autocomplete, aria-braillelabel, aria-brailleroledescription, aria-busy, aria-checked, aria-colcount, aria-colindex, aria-colindextext, aria-colspan, aria-controls, aria-current, aria-describedby, aria-description, aria-details, aria-disabled, aria-dropeffect, aria-errormessage, aria-expanded, aria-flowto, aria-grabbed, aria-haspopup, aria-hidden, aria-invalid, aria-keyshortcuts, aria-label, aria-labelledby, aria-level, aria-live, aria-multiline, aria-multiselectable, aria-orientation, aria-owns, aria-placeholder, aria-posinset, aria-pressed, aria-readonly, aria-relevant, aria-required, aria-roledescription, aria-rowcount, aria-rowindex, aria-rowindextext, aria-rowspan, aria-selected, aria-setsize, aria-sort, aria-valuemax, aria-valuemin, aria-valuenow, aria-valuetext, class, contenteditable, contextmenu, dir, draggable, dropzone, enterkeyhint, hidden, id, inert, inputmode, is, itemid, itemprop, itemref, itemscope, itemtype, lang, nonce, onabort, onblur, oncanplay, oncanplaythrough, onchange, onclick, oncontextmenu, oncopy, oncuechange, oncut, ondblclick, ondrag, ondragend, ondragenter, ondragleave, ondragover, ondragstart, ondrop, ondurationchange, onemptied, onended, onerror, onfocus, onformchange, onforminput, oninput, oninvalid, onkeydown, onkeypress, onkeyup, onload, onloadeddata, onloadedmetadata, onloadend, onloadstart, onlostpointercapture, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup, onmousewheel, onpaste, onpause, onplay, onplaying, onpointercancel, ongotpointercapture, onpointerdown, onpointerenter, onpointerleave, onpointermove, onpointerout, onpointerover, onpointerup, onprogress, onratechange, onreadystatechange, onreset, onsearch, onscroll, onseeked, onseeking, onselect, onshow, onstalled, onsubmit, onsuspend, ontimeupdate, ontoggle, ontouchcancel, ontouchend, ontouchmove, ontouchstart, onvolumechange, onwaiting, onwheel, onauxclick, oncancel, onclose, oncontextlost, oncontextrestored, onformdata, onmouseenter, onmouseleave, onresize, onsecuritypolicyviolation, onslotchange, role, slot, spellcheck, style, tabindex, title, translate, xmlns, xml:base, xml:lang, xml:space

  Custom data-* attributes, where the first three characters of the value of star (*) after lower-casing do not equal xml and the value of star does not have a colon (:), equal-to (=), newline, solidus (/), space, tab, or any A-Z character, are also considered global and allowed in all elements.

5.3  CSS 2.1 properties accepting URLs

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5.4  Microsoft Windows 1252 character replacements

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  Key: d double, l left, q quote, r right, s. single

  Code-point (decimal) - hexadecimal value - replacement entity - represented character

  127 - 7f - (removed) - (not used)
  128 - 80 - &#8364; - euro
  129 - 81 - (removed) - (not used)
  130 - 82 - &#8218; - baseline s. q
  131 - 83 - &#402; - florin
  132 - 84 - &#8222; - baseline d q
  133 - 85 - &#8230; - ellipsis
  134 - 86 - &#8224; - dagger
  135 - 87 - &#8225; - d dagger
  136 - 88 - &#710; - circumflex accent
  137 - 89 - &#8240; - permile
  138 - 8a - &#352; - S Hacek
  139 - 8b - &#8249; - l s. guillemet
  140 - 8c - &#338; - OE ligature
  141 - 8d - (removed) - (not used)
  142 - 8e - &#381; - Z dieresis
  143 - 8f - (removed) - (not used)
  144 - 90 - (removed) - (not used)
  145 - 91 - &#8216; - l s. q
  146 - 92 - &#8217; - r s. q
  147 - 93 - &#8220; - l d q
  148 - 94 - &#8221; - r d q
  149 - 95 - &#8226; - bullet
  150 - 96 - &#8211; - en dash
  151 - 97 - &#8212; - em dash
  152 - 98 - &#732; - tilde accent
  153 - 99 - &#8482; - trademark
  154 - 9a - &#353; - s Hacek
  155 - 9b - &#8250; - r s. guillemet
  156 - 9c - &#339; - oe ligature
  157 - 9d - (removed) - (not used)
  158 - 9e - &#382; - z dieresis
  159 - 9f - &#376; - Y dieresis

5.5  URL format

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  An absolute URL has a protocol or scheme, a network location or hostname, and, optional path, parameters, query and fragment segments. Thus, an absolute URL has this generic structure:

    (scheme) : (//network location) /(path) ;(parameters) ?(query) #(fragment)

  The schemes can only contain letters, digits, +, . and -. Hostname is the portion after the // and up to the first / (if any; else, up to the end) when : is followed by a // (e.g., in; otherwise, it consists of everything after the : (e.g., in').

  Relative URLs do not have explicit schemes and network locations; such values are inherited from a base URL.

5.6  Brief on htmLawed code

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  Much of the code's logic and reasoning can be understood from the documentation above.

  The output of htmLawed is a text string containing the processed input. There is no custom error tracking.

  Function arguments for htmLawed are:

  *  $in - first argument; a text string; the input text to be processed. Any extraneous slashes added by PHP when magic quotes are enabled should be removed beforehand using PHP's stripslashes() function.

  *  $config - second argument; an associative array; optional; named $C within htmLawed code. The array has keys with names like balance and keep_bad, and the values, which can be boolean, string, or array, depending on the key, are read to accordingly set the configurable parameters (indicated by the keys). All configurable parameters receive some default value if the value to be used is not specified by the user through $config. Finalized $config is thus a filtered and possibly larger array.

  *  $spec - third argument; a text string; optional. The string has rules, written in an htmLawed-designated format, specifying element-specific attribute and attribute value restrictions. Function hl_spec() is used to convert the string to an associative-array, named $S within htmLawed code, for internal use. Finalized $spec is thus an array.

  Finalized $config and $spec are made global variables while htmLawed is at work. Values of any pre-existing global variables with same names are noted, and their values are restored after htmLawed finishes processing the input (to capture the finalized values, the show_settings parameter of $config should be used). Depending on $config, another global variable hl_Ids, to track id attribute values for uniqueness, may be set. Unlike the other two variables, this one is not reset (or unset) post-processing.

  Except for the main htmLawed() function, htmLawed's functions are name-spaced using the hl_ prefix. The functions and their roles are:

  *  hl_attributeValue - check attribute values against $spec rules
  *  hl_balance - balance tags and ensure proper nesting
  *  hl_commentCdata - handle CDATA sections and HTML comments
  *  hl_deprecatedElement - transform element tags
  *  hl_entity - handle character entities
  *  hl_regex - check syntax of a regular expression
  *  hl_spec - convert $spec value to one used internally
  *  hl_tag - handle element tags and attributes
  *  hl_tidy - compact/beautify HTML
  *  hl_url - check URL-containing values
  *  hl_version - report htmLawed version
  *  htmLawed - main function

  htmLawed() finalizes $spec (with the help of hl_spec()) and $config, and globalizes them. Finalization of $config involves setting default values if an inappropriate or invalid one is supplied. This includes calling hl_regex() to check well-formedness of regular expression patterns if such expressions are user-supplied through $config. htmLawed() then removes invalid characters like nulls and x01 and appropriately handles entities using hl_entity(). HTML comments and CDATA sections are identified and treated as per $config with the help of hl_commentCdata(). When retained, the < and > characters identifying them, and the <, > and & characters inside them, are replaced with control characters (code-points 1 to 5) till any tag balancing is completed.

  After this initial processing htmLawed() identifies tags using regex and processes them with the help of hl_tag() --  a large function that analyzes tag content, filtering it as per HTML standards, $config and $spec. Among other things, hl_tag() transforms deprecated elements using hl_deprecatedElement(), removes attributes from closing tags, checks attribute values as per $spec rules using hl_attributeValue(), and checks URL protocols using hl_url(). htmLawed() performs tag balancing and nesting checks with a call to hl_balance(), and optionally compacts/beautifies the output with proper white-spacing with a call to hl_tidy(). The latter temporarily replaces white-space, and <, > and & characters inside pre, script and textarea elements, and HTML comments and CDATA sections with control characters (code-points 1 to 5, and 7).

  htmLawed permits the use of custom code or hook functions at two stages. The first, called inside htmLawed(), allows the input text as well as the finalized $config and $spec values to be altered right after the initial processing (see section 3.7). The second is called by hl_tag() once the tag content is finalized (see section 3.4.9).

  The functionality of htmLawed is dictated by the external HTML standards. The code of htmLawed is thus written for a clear-cut aim, with not much concern for tweaking by other developers. The code is only minimally annotated with comments -- it is not meant to instruct. PHP developers familiar with the HTML specifications will see the logic, and others can always refer to the htmLawed documentation.

HTM version of htmLawed_README.txt generated on 05 Aug, 2023 using rTxt2htm from PHP Labware