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RFC 7764

Guidance on Markdown: Design Philosophies, Stability Strategies, and Select Registrations

Pages: 28
Informational

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Internet Engineering Task Force (IETF)                        S. Leonard
Request for Comments: 7764                                 Penango, Inc.
Category: Informational                                       March 2016
ISSN: 2070-1721


                         Guidance on Markdown:
  Design Philosophies, Stability Strategies, and Select Registrations

Abstract

This document elaborates upon the text/markdown media type for use with Markdown, a family of plain-text formatting syntaxes that optionally can be converted to formal markup languages such as HTML. Background information, local storage strategies, and additional syntax registrations are supplied. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7764. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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Table of Contents

1. Dive into Markdown . . . . . . . . . . . . . . . . . . . . . . 3 1.1. On Formats . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Markdown Design Philosophy . . . . . . . . . . . . . . . . 4 1.3. Uses of Markdown . . . . . . . . . . . . . . . . . . . . . 5 1.4. Uses of Labeling Markdown Content as text/markdown . . . . 6 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Strategies for Preserving Media Type and Parameters . . . . . 7 2.1. Map to Filename and Attributes . . . . . . . . . . . . . . 7 2.2. Store Headers in Adjacent File . . . . . . . . . . . . . . 8 2.3. "Arm" Content with MIME Headers . . . . . . . . . . . . . . 8 2.4. Create a Local Batch Script . . . . . . . . . . . . . . . . 9 2.5. Process the Markdown in Advance . . . . . . . . . . . . . . 9 2.6. Rely on Context . . . . . . . . . . . . . . . . . . . . . . 9 2.7. Specific Strategies . . . . . . . . . . . . . . . . . . . . 9 2.7.1. Subversion . . . . . . . . . . . . . . . . . . . . . . 9 2.7.2. Git . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3. Registration Templates for Common Markdown Syntaxes . . . . . 10 3.1. MultiMarkdown . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. GitHub-Flavored Markdown . . . . . . . . . . . . . . . . . 11 3.3. Pandoc . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4. Fountain (Fountain.io) . . . . . . . . . . . . . . . . . . 14 3.5. CommonMark . . . . . . . . . . . . . . . . . . . . . . . . 14 3.6. kramdown-rfc2629 (Markdown for RFCs) . . . . . . . . . . . 15 3.7. rfc7328 (Pandoc2rfc) . . . . . . . . . . . . . . . . . . . 16 3.8. PHP Markdown Extra . . . . . . . . . . . . . . . . . . . . 16 4. Examples for Common Markdown Syntaxes . . . . . . . . . . . . 17 4.1. MultiMarkdown . . . . . . . . . . . . . . . . . . . . . . . 17 4.2. GitHub Flavored Markdown . . . . . . . . . . . . . . . . . 18 4.3. Pandoc . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.4. Fountain (Fountain.io) . . . . . . . . . . . . . . . . . . 20 4.5. CommonMark . . . . . . . . . . . . . . . . . . . . . . . . 21 4.6. kramdown-rfc2629 (Markdown for RFCs) . . . . . . . . . . . 22 4.7. rfc7328 (Pandoc2rfc) . . . . . . . . . . . . . . . . . . . 25 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 26 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 7.1. Normative References . . . . . . . . . . . . . . . . . . . 26 7.2. Informative References . . . . . . . . . . . . . . . . . . 26 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Dive into Markdown

This document serves as an informational companion to [RFC7763], the text/markdown media type registration. It should be considered jointly with [RFC7763]. "Sometimes the truth of a thing is not so much in the think of it, but in the feel of it." -- Stanley Kubrick

1.1. On Formats

In computer systems, textual data is stored and processed using a continuum of techniques. On the one end is plain text: computer- encoded text that consists only of a sequence of code points from a given standard, with no other formatting or structural information [UNICODE]. Plain text provides /some/ fixed facilities for formatting instructions (namely, codes in the character set that have meanings other than "represent this character graphically on the output medium"); however, these facilities are not particularly extensible. Compare with Section 4.2.1 of [RFC6838]. Applications may neuter the effects of these special characters by prohibiting them or by ignoring their dictated meanings, as is the case with how modern applications treat most control characters in US-ASCII. On this end, any text reader or editor that interprets the character set can be used to see or manipulate the text. If some characters are corrupted, the corruption is unlikely to affect the ability of a computer system to process the text (even if the human meaning is changed). On the other end is binary data: a sequence of bits intended for some computer application to interpret and act upon. Binary formats are flexible in that they can store non-textual data efficiently (perhaps storing no text at all, or only storing certain kinds of text for very specialized purposes). Binary formats require an application to be coded specifically to handle the format; no partial interoperability is possible. Furthermore, if even one bit is corrupted in a binary format, it may prevent an application from processing any of the data correctly. Between these two extremes lies formatted text, i.e., text that includes non-textual information coded in a particular way, that affects the interpretation of the text by computer programs. Formatted text is distinct from plain text and binary data in that the non-textual information is encoded into textual characters that are assigned specialized meanings not defined by the character set. With a regular text editor and a standard keyboard (or other standard input mechanism), a user can enter these textual characters to express the non-textual meanings. For example, a character like "<"
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   no longer means "LESS-THAN SIGN"; it means the start of a tag or
   element that affects the document in some way.

   On the formal end of the formatted text spectrum is markup, a family
   of languages for annotating a document in such a way that the
   annotations are syntactically distinguishable from the text.  Markup
   languages are (reasonably) well-specified and tend to follow (mostly)
   standardized syntax rules.  Examples of markup languages include
   Standard Generalized Markup Language (SGML), HTML, XML, and LaTeX.
   Standardized rules lead to interoperability between markup
   processors, but a skill requirement for new (human) users of the
   language that they learn these rules in order to do useful work.
   This imposition makes markup less accessible for non-technical users
   (i.e., users who are unwilling or unable to invest in the requisite
   skill development).

     informal        /---------formatted text----------\        formal
     <------v-------------v-------------v-----------------------v---->
      plain text     informal markup   formal markup    binary format
                     (Markdown)        (HTML, XML, etc.)

    Figure 1: Degrees of Formality in Data-Storage Formats for Text

   On the informal end of the spectrum are lightweight markup languages.
   In comparison with formal markup like XML, lightweight markup uses
   simple syntax, and is designed to be easy for humans to enter with
   basic text editors.  Markdown, the subject of this document, is an
   /informal/ plain-text formatting syntax that is intentionally
   targeted at non-technical users (i.e., users upon whom little to no
   skill development is imposed) using unspecialized tools (i.e., text
   boxes).  Jeff Atwood once described these informal markup languages
   as "humane" [HUMANE].

1.2. Markdown Design Philosophy

Markdown specifically is a family of syntaxes that are based on the original work of John Gruber with substantial contributions from Aaron Swartz, released in 2004 [MARKDOWN]. Since its release, a number of web or web-facing applications have incorporated Markdown into their text-entry systems, frequently with custom extensions. Fed up with the complexity and security pitfalls of formal markup languages (e.g., HTML5) and proprietary binary formats (e.g., commercial word-processing software), yet unwilling to be confined to the restrictions of plain text, many users have turned to Markdown for document processing. Whole toolchains now exist to support Markdown for online and offline projects.
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   Informality is a bedrock premise of Gruber's design.  Gruber created
   Markdown after disastrous experiences with strict XML and XHTML
   processing of syndicated feeds.  In Mark Pilgrim's "thought
   experiment", several websites went down because one site included
   invalid XHTML in a blog post, which was automatically copied via
   trackbacks across other sites [DIN2MD].  These scenarios led Gruber
   to believe that clients (e.g., web browsers) SHOULD try to make sense
   of data that they receive, rather than rejecting data simply because
   it fails to adhere to strict, unforgiving standards.  (In [DIN2MD],
   Gruber compared Postel's Law [RFC793] with the XML standard, which
   says: "Once a fatal error is detected [...] the processor MUST NOT
   continue normal processing" [XML1.0-5].) As a result, there is no
   such thing as "invalid" Markdown, there is no standard demanding
   adherence to the Markdown syntax, and there is no governing body that
   guides or impedes its development.  If the Markdown syntax does not
   result in the "right" output (defined as output that the author
   wants, not output that adheres to some dictated system of rules),
   Gruber's view is that the author either should keep on experimenting
   or should change the processor to address the author's particular
   needs (see [MARKDOWN] Readme and [MD102b8] perldoc; see also
   [CATPICS]).

1.3. Uses of Markdown

Since its introduction in 2004, Markdown has enjoyed remarkable success. Markdown works for users for three key reasons. First, the markup instructions (in text) look similar to the markup that they represent; therefore, the cognitive burden to learn the syntax is low. Second, the primary arbiter of the syntax's success is *running code*. The tool that converts the Markdown to a presentable format, and not a series of formal pronouncements by a standards body, is the basis for whether syntactic elements matter. Third, Markdown has become something of an Internet meme [INETMEME], in that Markdown gets received, reinterpreted, and reworked as additional communities encounter it. There are communities that are using Markdown for scholarly writing [OCCASION], for screenplays [FOUNTAIN], and even for mathematical formulae [MATHDOWN]. Clearly, a screenwriter has no use for specialized Markdown syntax for mathematicians; likewise, mathematicians do not need to identify characters or props in common ways. The overall gist is that all of these communities can take the common elements of Markdown (which are rooted in the common elements of HTML circa 2004) and build on them in ways that best fit their needs.
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1.4. Uses of Labeling Markdown Content as text/markdown

The primary purpose of an Internet media type is to label "content" on the Internet, as distinct from "files". Content is any computer- readable format that can be represented as a primary sequence of octets, along with type-specific metadata (parameters) and type- agnostic metadata (protocol dependent). From this description, it is apparent that appending ".markdown" to the end of a filename is not a sufficient means to identify Markdown. Filenames are properties of files in file systems, but Markdown frequently exists in databases or content management systems (CMSes) where the file metaphor does not apply. One CMS [RAILFROG] uses media types to select appropriate processing, so a media type is necessary for the safe and interoperable use of Markdown. Unlike complete HTML documents, [MDSYNTAX] provides no means to include metadata in the content stream. Several derivative flavors have invented metadata incorporation schemes (e.g., [MULTIMD]), but these schemes only address specific use cases. In general, the metadata must be supplied via supplementary means in an encapsulating protocol, format, or convention. The relationship between the content and the metadata is not directly addressed here or in [RFC7763]; however, by identifying Markdown with a media type, Markdown content can participate as a first-class citizen with a wide spectrum of metadata schemes. Finally, registering a media type through the IETF process is not trivial. Markdown can no longer be considered a "vendor"-specific innovation, but the registration requirements even in the vendor tree have proven to be overly burdensome for most Markdown implementers. Moreover, registering hundreds of Markdown variants with distinct media types would impede interoperability: virtually all Markdown content can be processed by virtually any Markdown processor, with varying degrees of success. The goal of [RFC7763] is to reduce all of these burdens by having one media type that accommodates diversity and eases registration.

1.5. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Since Markdown signifies a family of related formats with varying degrees of formal documentation and implementation, this specification uses the term "variant" to identify such formats.
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2. Strategies for Preserving Media Type and Parameters

The purpose of this document and [RFC7763] is to promote interoperability between different Markdown-related systems, preserving the author's intent. While [MARKDOWN] was designed by Gruber in 2004 as a simple way to write blog posts and comments, as of 2014 Markdown and its derivatives are rapidly becoming the formats of record for many communities and use cases. While an individual member of (or software tool for) a community can probably look at some "Markdown" and declare its meaning intuitively obvious, software systems in different communities (or different times) need help. [MDSYNTAX] does not have a signaling mechanism like <!DOCTYPE>, so tagging Markdown internally is simply out of the question. Once tags or metadata are introduced, the content is no longer "just" Markdown. Some commentators have suggested that an in-band signaling mechanism, such as in Markdown link definitions at the top of the content, could be used to signal the variant. Unfortunately, this signaling mechanism is incompatible with other Markdown variants (e.g., [PANDOC]) that expect their own kinds of metadata at the top of the file. Markdown content is just a stream of text; the semantics of that text can only be furnished by context. The media type and variant parameter in [RFC7763] furnish this missing context, while allowing for additional extensibility. This section covers strategies for how an application might preserve metadata when it leaves the domain of IETF protocols. [RFC7763] only defines two parameters: the charset parameter (required for all text/* media types) and the variant parameter. [RFC6657] provides guidance on character-set parameter handling. The variant parameter provides a simple identifier -- nothing less or more. Variants are allowed to define additional parameters when sent with the text/markdown media type; the variant can also introduce control information into the textual content stream (such as via a metadata block). Neither [RFC7763] nor this specification recommend any particular approach. However, the philosophy behind [RFC7763] is to preserve formats rather than create new ones, since supporting existing toolchains is more realistic than creating novel ones that lack traction in the Markdown community.

2.1. Map to Filename and Attributes

This strategy is to map the media type, variant, and parameters to "attributes" or "forks" in the local convention. Firstly, Markdown content saved to a file should have an appropriate file extension ending in .md or .markdown, which serves to disambiguate it from other kinds of files. The character repertoire of variant
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   identifiers in [RFC7763] is designed to be compatible with most
   filename conventions.  Therefore, a recommended strategy is to record
   the variant identifier as the prefix to the file extension.  For
   example, for [PANDOC] content, a file could be named
   "example.pandoc.markdown".

   Many filesystems are case-sensitive or case-preserving; however, file
   extensions tend to be all lowercase.  This document takes no position
   on whether variant identifiers should be case-preserved or all
   lowercase when Markdown content is written to a file.  However, when
   the variant identifier is read to influence operational behavior, it
   needs to be compared case-insensitively.

   Many modern filesystems support "extended attributes", "alternate
   data streams", or "resource forks".  Some version control systems
   support named properties.  If the variant defines additional
   parameters, these parameters should be stored in these resources,
   where the parameter name includes the name of the resource, and the
   parameter value is the value of the resource (data in the resource),
   preferably UTF-8 encoded (unless the parameter definition explicitly
   defines a different encoding or repertoire).  The variant identifier
   itself should be stored in a resource with a name including the term
   "variant" (possibly including other decorations to avoid namespace
   collisions).

2.2. Store Headers in Adjacent File

This strategy is to save the Markdown content in a first file and to save the metadata (specifically the Content-Type header) in a second file with a filename that is rationally related to the first filename. For example, if the first file is named "readme.markdown", the second file could be named "readme.markdown.headers". (If stored in a database, the analogy would be to store the metadata in a second table with a field that is a key to the first table.) This header file has the media type message/global-headers [RFC6533] (".u8hdr" suggestion notwithstanding).

2.3. "Arm" Content with MIME Headers

This strategy is to save the Markdown content along with its headers in a file, "arming" the content by prepending the MIME headers (specifically the Content-Type header). It should be appreciated that the file is no longer a "Markdown file"; rather, it is an Internet Message Format file (e.g., [RFC5322]) with a Markdown content part. Therefore, the file should have an Internet message extension (e.g., ".eml", ".msg", or ".u8msg"), not a Markdown extension (e.g., ".md" or ".markdown").
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2.4. Create a Local Batch Script

This strategy is to translate the processing instructions inferred from the Content-Type and other parameters (e.g., Content- Disposition) into a sequence of commands in the local convention, storing those commands in a batch script. For example, when a MIME- aware client stores some Markdown to disk, the client can save a Makefile in the same directory with commands that are appropriate (and safe) for the local system.

2.5. Process the Markdown in Advance

This strategy is to process the Markdown into the formal markup, before a recipient receives it; this eliminates ambiguities. Once the Markdown is processed into (for example) valid XHTML, an application can save a file as "doc.xhtml" or can send MIME content as application/xhtml+xml with no further loss of metadata. While unambiguous, this process may not be reversible.

2.6. Rely on Context

This last strategy is to use or create context to determine how to interpret the Markdown. For example, Markdown content that is of the Fountain.io type [FOUNTAIN] could be saved with the filename "script.fountain" instead of "script.markdown". Alternatively, scripts could be stored in a "/screenplays" directory while other kinds of Markdown could be stored elsewhere. For reasons that should be intuitively obvious, this method is the most error-prone. "Context" can be easily lost over time, and the trend of passing Markdown between systems -- taking them *out* of context -- is increasing.

2.7. Specific Strategies

2.7.1. Subversion

This subsection covers a preservation strategy in Subversion [SVN], a common client-server version control system. Subversion supports named properties. The "svn:mime-type" property duplicates the entire Content-Type header, so parameters SHOULD be stored there (Section 2.1). The filename SHOULD be consistent with this Content-Type header, i.e., the extension SHOULD be the variant identifier plus ".markdown" (Section 2.1).
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2.7.2. Git

This subsection covers a preservation strategy in Git [GIT], a common distributed version control system. Versions of Git as of the time of this writing do not support arbitrary metadata storage; however, third-party projects add this support. If Git is used without a metadata storage service, then a reasonable strategy is to include the variant identifier in the filename (Section 2.1). The default text encoding SHOULD be UTF-8. For other or different properties, a header file SHOULD be recorded alongside the Markdown file (Section 2.2). If a metadata storage service is used with Git, then use a convention that is most analogous to the service. For example, the "metastore" project emulates extended attributes (xattrs) of a POSIX-like system, so whatever "xattr" methodology is developed would be usable with metastore and Git.

3. Registration Templates for Common Markdown Syntaxes

The purpose of this section is to register certain syntaxes in the "Markdown Variants" registry [RFC7763] because they illustrate particularly interesting use cases or are broadly applicable to the Internet community; thus, these syntaxes would benefit from the level of review associated with publication as IETF documents.

3.1. MultiMarkdown

Identifier: MultiMarkdown Name: MultiMarkdown Description: MultiMarkdown (MMD) is a superset of "Original". It adds multiple syntax features (tables, footnotes, and citations, to name a few) and is intended to output to various formats. Additionally, it builds in "smart" typography for various languages (proper left- and right-sided quotes, for example).
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   Additional Parameters:
      options: String with zero or more of the following tokens
      delimited by whitespace (WSP):

            "memoir" / "beamer"
            "full" / "snippet"
            "process-html"
            "random-footnote-identifiers"
            "accept"
            "reject"
            "nosmart"
            "nonotes"
            "nolabels"
            "nomask"

            The meanings of these tokens are defined in the
            MultiMarkdown documentation.

   References:
      <http://fletcher.github.io/MultiMarkdown-4/syntax>

   Contact Information:
      (individual) Fletcher T. Penney <fletcher@fletcherpenney.net>
                   <http://fletcherpenney.net/multimarkdown/>

3.2. GitHub Flavored Markdown

Identifier: GFM Name: GitHub Flavored Markdown Description: "Original" with the following differences: 1. Multiple underscores in words 2. URL (URI) autolinking 3. Strikethrough 4. Fenced code blocks 5. Syntax highlighting 6. Tables (- for rows; | for columns; : for alignment) 7. Only some HTML allowed; sanitization is integral to the format References: <https://help.github.com/articles/github-flavored-markdown/> <https://github.com/github/markup/tree/master#html-sanitization> Contact Information: (corporate) GitHub, Inc. <https://github.com/contact>
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3.3. Pandoc

Identifier: pandoc Name: Pandoc Description: Markdown is designed to be easy to write and to read: the content should be publishable as-is, as plain text, without looking like it has been marked up with tags or formatting instructions. Yet whereas "Original" has HTML generation in mind, pandoc is designed for multiple output formats. Thus, while pandoc allows the embedding of raw HTML, it discourages it, and provides other, non- HTMLish ways of representing important document elements like definition lists, tables, mathematics, and footnotes. Additional Parameters: extensions: String with an optional starting syntax token, followed by a "+" and "-" delimited list of extension tokens. "+" preceding an extension token turns the extension on; "-" turns the extension off. The starting syntax tokens are "markdown", "markdown_strict", "markdown_phpextra", and "markdown_github". If no starting syntax token is given, "markdown" is assumed. The extension tokens include: Extensions to turn off (on by default): escaped_line_breaks blank_before_header header_attributes auto_identifiers implicit_header_references blank_before_blockquote fenced_code_blocks fenced_code_attributes line_blocks fancy_lists startnum definition_lists example_lists table_captions simple_tables multiline_tables grid_tables pipe_tables pandoc_title_block
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                  yaml_metadata_block
                  all_symbols_escapable
                  intraword_underscores
                  strikeout
                  superscript
                  subscript
                  inline_code_attributes
                  tex_math_dollars
                  raw_html
                  markdown_in_html_blocks
                  native_divs
                  native_spans
                  raw_tex
                  latex_macros
                  implicit_figures
                  footnotes
                  inline_notes
                  citations

                  Extensions to turn on (off by default):

                  lists_without_preceding_blankline
                  hard_line_breaks
                  ignore_line_breaks
                  tex_math_single_backslash
                  tex_match_double_backslash
                  markdown_attribute
                  mmd_title_block
                  abbreviations
                  autolink_bare_uris
                  ascii_identifiers
                  link_attributes
                  mmd_header_identifiers
                  compact_definition_lists

   Fragment Identifiers:
      Pandoc defines fragment identifiers using the <id> in the
      {#<id> .class ...} production (PHP Markdown Extra attribute
      block).  This syntax works for Header Identifiers and Code Block
      Identifiers.

   References:
      <http://johnmacfarlane.net/pandoc/README.html#pandocs-markdown>

   Contact Information:
      (individual) Prof. John MacFarlane <jgm@berkeley.edu>
                   <http://johnmacfarlane.net/>
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3.4. Fountain (Fountain.io)

Identifier: Fountain Name: Fountain Description: Fountain is a simple markup syntax for writing, editing, and sharing screenplays in plain, human-readable text. Fountain allows you to work on your screenplay anywhere, on any computer or tablet, using any software that edits text files. Fragment Identifiers: See <http://fountain.io/syntax#section-titlepage> and <http://fountain.io/syntax#section-sections>. In the following fragment identifiers, the <key> and <sec*> productions MUST have "/" characters percent-encoded. #/ Title Page (acts as metadata). #/<key> Title Page; <key> is the key string. #<sec1> *("/" <secn>) Section or subsection. The <sec1>..<secn> productions are the text of the Section line, with whitespace trimmed from both ends. Subsections (sections with multiple # characters at the beginning of the line in the source) are addressed hierarchically by preceding the subsection with higher-order sections. If the section hierarchy "skips", e.g., # to ###, use a blank section name, e.g., #Section/ACT%20I//PATIO%20SCENE. References: <http://fountain.io/syntax> Contact Information: (individual) Stu Maschwitz <http://prolost.com/> (individual) John August <http://johnaugust.com/>

3.5. CommonMark

Identifier: CommonMark Name: CommonMark Description: CommonMark is a standard, unambiguous syntax specification for Markdown, along with a suite of comprehensive tests to validate
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      Markdown implementations against this specification.  The
      maintainers believe that CommonMark is necessary, even essential,
      for the future of Markdown.

      Compared to "Original", CommonMark is much longer and in a few
      instances contradicts "Original" based on seasoned experience.
      Although CommonMark specifically does not mandate any particular
      encoding for the input content, CommonMark draws in more of
      Unicode, UTF-8, and HTML (including HTML5) than "Original".

      This registration always refers to the latest version or an
      unspecified version (receiver's choice).  Version 0.13 of the
      CommonMark specification was released 2014-12-10.

   References:
      <http://spec.commonmark.org/>

   Contact Information:
      (individual) John MacFarlane <jgm@berkeley.edu>
      (individual) David Greenspan <david@meteor.com>
      (individual) Vicent Marti <vicent@github.com>
      (individual) Neil Williams <neil@reddit.com>
      (individual) Benjamin Dumke-von der Ehe <ben@stackexchange.com>
      (individual) Jeff Atwood <jatwood@codinghorror.com>

3.6. kramdown-rfc2629 (Markdown for RFCs)

Identifier: kramdown-rfc2629 Name: Markdown for RFCs Description: kramdown is a markdown parser by Thomas Leitner; it has a number of backends for generating HTML, LaTeX, and Markdown again. kramdown-rfc2629 is an additional backend to that: It allows the generation of XML2RFC XML markup (originally known as markup that is RFC 2629 compliant, now documented in RFC 7749). References: <https://github.com/cabo/kramdown-rfc2629> Contact Information: (individual) Carsten Bormann <cabo@tzi.org>
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3.7. rfc7328 (Pandoc2rfc)

Identifier: rfc7328 Name: Pandoc2rfc Description: Pandoc2rfc allows authors to write in "pandoc" that is then transformed to XML and given to xml2rfc. The conversions are, in a way, amusing, as we start off with (almost) plain text, use elaborate XML, and end up with plain text again. References: RFC 7328 <https://github.com/miekg/pandoc2rfc> Contact Information: (individual) R. (Miek) Gieben <miek@google.com>

3.8. PHP Markdown Extra

Identifier: Extra Name: Markdown Extra Description: Markdown Extra is an extension to PHP Markdown implementing some features currently not available with the plain Markdown syntax. Markdown Extra is available as a separate parser class in PHP Markdown Lib. Other implementations include Maruku (Ruby) and Python Markdown. Markdown Extra is supported in several content management systems, including Drupal, TYPO3, and MediaWiki. Fragment Identifiers: Markdown Extra defines fragment identifiers using the <id> in the {#<id> .class ...} production (attribute block). This syntax works for headers, fenced code blocks, links, and images. References: <https://michelf.ca/projects/php-markdown/extra/> Contact Information: (individual) Michel Fortin <michel.fortin@michelf.ca>
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4. Examples for Common Markdown Syntaxes

This section provides examples of the variants in Section 3.

4.1. MultiMarkdown

Title: Example of MultiMarkdown Keywords: IETF, example, footnotes # MultiMarkdown Example # MultiMarkdown supports several cool features, as well as several output formats: * HTML * PDF * OpenDocument * OPML * LaTeX ## Footnotes ## Footnotes are described in the MultiMarkdown Syntax Guide.[^somesamplefootnote] [^somesamplefootnote]: Here is the text of the footnote itself. Figure 1: MultiMarkdown Example
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4.2. GitHub-Flavored Markdown

# Start Out # GFM is like regular Markdown with a few extra features. For example, http://www.example.com/ will get auto-linked. ~~This is strike-through text, demarked by the double tildes.~~ ``` function test() { return "notice this feature?"); } ``` # Table Alignments # | Left | Center | Right | |:--------- |:-------:| ------:| | cats | Paxton | $1600 | | dogs | Ruff | $30 | | zebras | Stripes | $20900 | Figure 2: GitHub Flavored Markdown Example
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4.3. Pandoc

% Pandoc User's Guide % John MacFarlane % August 30, 2014 Synopsis {#syn} ======== pandoc [*options*] [*input-file*]... Description {#desc} =========== Pandoc is a [Haskell] library for converting from one markup format to another, and a command-line tool that uses this library. #### Extension: `header_attributes` #### {#ext-header-attributes} Headers can be assigned attributes using this syntax at the end of the line containing the header text: {#identifier .class .class key=value key=value} Thus, for example, the following headers will all be assigned the identifier `foo`: # My header {#foo} ## My header ## {#foo} My other header {#foo} --------------- Figure 3: Pandoc Example
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4.4. Fountain (Fountain.io)

INT. BOXCAR - MOVING - DAY ?AGENT MORTIMER lies bleeding in the corner. The car ROCKS gently. Mortimer pulls out his cell phone and dials. MORTIMER? Come on. Pick up. CUT TO:? ext. hotel bar - day? A fiercely gorgeous brunette sips the last of something from a rocks glass. This is REBECCA. Behind her, a dark FIGURE approaches. She seems not to notice. REBECCA?(to Bartender) Rittenhouse, neat. FIGURE (O.S.) ^ Ritenhouse, neat. She turns to find the source of the voice. FIGURE Excellent choice. Before she can reply, her phone RINGS.? > INTERCUT WITH:? .THE BOXCAR Where MORTIMER is just barely holding on to life. Figure 4: Fountain Example
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4.5. CommonMark

CommonMark is like Markdown. Here are some entity names that you can use with CommonMark: `&nbsp; &amp; &copy; &AElig; &Dcaron; &frac34; &HilbertSpace; &DifferentialD; &ClockwiseContourIntegral;` You can see more at [the CommonMark website](http://commonmark.org/ "CommonMark"). - foo *** - bar Tildes can be used for fenced code blocks: ~~~ < > ~~~ Figure 5: CommonMark Example
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4.6. kramdown-rfc2629 (Markdown for RFCs)

--- title: STUN/TURN using PHP in Despair abbrev: STuPiD-excerpt docname: draft-hartke-xmpp-stupid-excerpt-00 date: 2009-07-05 category: info ipr: trust200902 area: General workgroup: XMPP Working Group keyword: Internet-Draft stand_alone: yes pi: [toc, sortrefs, symrefs] author: - ins: K. Hartke name: Klaus Hartke email: example@tzi.org normative: RFC2119: informative: RFC5389: STUNT: target: http://www.example.com/oob title: STUNT & out-of-band channels author: name: Robbie Hanson ins: R. Hanson date: 2007-09-17 --- abstract NAT (Network Address Translator) Traversal may require TURN (Traversal Using Relays around NAT) functionality in certain cases that are not unlikely to occur. There is little incentive to deploy TURN servers, except by those who need them&#x2014;who may not be in a position to deploy a new protocol on an Internet-connected node, in particular not one with deployment requirements as high as those of TURN. --- middle
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Introduction        {#problems}
============

"STUN/TURN using PHP in Despair" is a highly deployable protocol for
obtaining TURN-like functionality, while also providing the most
important function of STUN {{RFC5389}}.

The Need for Standardization   {#need}
----------------------------

Having one standard form of STuPiD service instead of one specific to
each kind of client also creates an incentive for optimized
implementations.

~~~~~~~~~~


        STuPiD   ```````````````````````````````,
        Script   <----------------------------. ,
                                              | ,
          ^ ,                                 | ,
          | ,                                 | ,
    (1)   | ,                                 | ,  (3)
    POST  | ,                                 | ,  GET
          | ,                                 | ,
          | v                                 | v

        Peer A   ----------------------->   Peer B
                           (2)
                       out-of-band
                       Notification
~~~~~~~~~~
{: #figops title="STuPiD Protocol Operation"}

Terminology          {#Terminology}
-----------
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
{{RFC2119}} and indicate requirement levels for compliant STuPiD
implementations.

--- back

Sample Implementation     {#impl}
=====================

~~~~~~~~~~
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<?php
header("Cache-Control: no-cache, must-revalidate");
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");
header("Content-Type: application/octet-stream");

?>
~~~~~~~~~~
{: #figimpl title="STuPiD Sample Implementation"}

                  Figure 6: Markdown for RFCs Example
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4.7. rfc7328 (Pandoc2rfc)

Pandoc2rfc expects multiple files as input. The following figure is example of "middle.mkd". # Introduction <?rfc toc="yes"?> <?rfc symrefs="yes"?> <?rfc sortrefs="yes"?> <?rfc subcompact="no"?> <?rfc compact="yes"?> <?rfc comments="yes"?> This document presents a technique for using Pandoc syntax as a source format for documents in the Internet-Drafts (I-Ds) and Request for Comments (RFC) series. This version is adapted to work with `xml2rfc` version 2.x. Pandoc is an "almost plain text" format and therefore particularly well suited for editing RFC-like documents. > Note: this document is typeset in Pandoc. > NB: this is mostly text to test Pandoc2rfc, the canonical > documentation is [RFC 7328][p2r]. [p2r]: http://www.rfc-editor.org/info/rfc7328 # Pandoc to RFC > Pandoc2rfc -- designed to do the right thing, until it doesn't. When writing [](#RFC4641) we directly wrote the XML. Needless to say it was tedious even though the XML of [xml2rfc](http://xml2rfc.ietf.org/) is very "light". The [latest version of xml2rfc version 2 can be found here](http://pypi.python.org/pypi/xml2rfc/). Figure 7: Pandoc2rfc Example (middle.mkd)

5. IANA Considerations

IANA has registered the syntaxes specified in Section 3 in the "Markdown Variants" registry.
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6. Security Considerations

See the respective syntax descriptions and output media type registrations for their respective security considerations.

7. References

7.1. Normative References

[MARKDOWN] Gruber, J., "Daring Fireball: Markdown", December 2004, <http://daringfireball.net/projects/markdown/>. [MDSYNTAX] Gruber, J., "Daring Fireball: Markdown Syntax Documentation", <http://daringfireball.net/projects/markdown/syntax>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, <http://www.rfc-editor.org/info/rfc5322>. [RFC6657] Melnikov, A. and J. Reschke, "Update to MIME regarding "charset" Parameter Handling in Textual Media Types", RFC 6657, DOI 10.17487/RFC6657, July 2012, <http://www.rfc-editor.org/info/rfc6657>. [RFC7763] Leonard, S., "The text/markdown Media Type", RFC 7763, DOI 10.17487/RFC7763, March 2016, <http://www.rfc-editor.org/info/rfc7763>.

7.2. Informative References

[UNICODE] The Unicode Consortium, "The Unicode Standard, Version 8.0", (Mountain View, CA: The Unicode Consortium, 2015. ISBN 978-1-936213-10-8), <http://www.unicode.org/versions/Unicode8.0.0/>. [HUMANE] Atwood, J., "Is HTML a Humane Markup Language?", May 2008, <http://blog.codinghorror.com/ is-html-a-humane-markup-language/>. [DIN2MD] Gruber, J., "Dive Into Markdown", March 2004, <http://daringfireball.net/2004/03/dive_into_markdown>.
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   [MD102b8]  Gruber, J., "Subject: [ANN] Markdown.pl 1.0.2b8", message
              to the markdown-discuss mailing list, 9 May 2007,
              <http://six.pairlist.net/pipermail/markdown-discuss/
              2007-May/000615.html>,
              <http://daringfireball.net/projects/downloads/
              Markdown_1.0.2b8.tbz>.

   [CATPICS]  Gruber, J. and M. Arment, "The Talk Show: Ep. 88: 'Cat
              Pictures' (Side 1)", July 2014,
              <http://daringfireball.net/thetalkshow/2014/07/19/ep-088>.

   [INETMEME] Solon, O., "Richard Dawkins on the internet's hijacking of
              the word 'meme'", June 2013,
              <http://www.wired.co.uk/news/archive/2013-06/20/
              richard-dawkins-memes>,
              <http://www.webcitation.org/6HzDGE9Go>.

   [MULTIMD]  Penney, F., "MultiMarkdown",
              <http://fletcherpenney.net/multimarkdown/>.

   [PANDOC]   MacFarlane, J., "Pandoc",
              <http://johnmacfarlane.net/pandoc/>.

   [RAILFROG] Railfrog Team, "Railfrog", April 2009,
              <http://railfrog.com/>.

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC6533]  Hansen, T., Ed., Newman, C., and A. Melnikov,
              "Internationalized Delivery Status and Disposition
              Notifications", RFC 6533, DOI 10.17487/RFC6533, February
              2012, <http://www.rfc-editor.org/info/rfc6533>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [XML1.0-5] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", W3C Recommendation REC-xml-20081126, November
              2008, <http://www.w3.org/TR/2008/REC-xml-20081126>.
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   [OCCASION] Shieber, S., "Switching to Markdown for scholarly article
              production", August 2014,
              <http://blogs.law.harvard.edu/pamphlet/2014/08/29/
              switching-to-markdown-for-scholarly-article-production/>.

   [FOUNTAIN] Maschwitz, S. and J. August, "Fountain | A markup language
              for screenwriting.", <http://fountain.io/>.

   [MATHDOWN] Cherniavsky-Paskin, B., "math in markdown",
              <https://github.com/cben/mathdown/wiki/math-in-markdown>.

   [SVN]      Apache Subversion, December 2015,
              <https://subversion.apache.org/>.

   [GIT]      Git, <http://git-scm.com/>.

Author's Address

Sean Leonard Penango, Inc. 5900 Wilshire Boulevard 21st Floor Los Angeles, CA 90036 United States EMail: dev+ietf@seantek.com URI: http://www.penango.com/