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Keywords: text/markdown
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:
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: `
& © Æ Ď ¾ ℋ ⅆ
∲`
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—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/
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