XEP-0393: Message Styling

Abstract
This specification defines a formatted text syntax for use in instant messages with simple text styling.
Author
Sam Whited
Copyright
© 1999 – 2021 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status

Draft

NOTICE: The protocol defined herein is a Draft Standard of the XMPP Standards Foundation. Implementations are encouraged and the protocol is appropriate for deployment in production systems, but some changes to the protocol are possible before it becomes a Final Standard.
Type
Standards Track
Version
1.1.1 (2021-04-04)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Draft
  4. Final

1. Introduction

Historically, XMPP has had no system for simple text styling. Instead, specifications like XHTML-IM (XEP-0071) [1] that require full layout engines have been used, leading to numerous security issues with implementations. Some entities have also performed their own styling based on identifiers in the body. While this has worked well in the past, it is not interoperable and leads to entities each supporting their own informal styling languages.

This specification aims to provide a single, interoperable formatted text syntax that can be used by entities that do not require full layout engines.

2. Requirements

3. Use Cases

4. Glossary

Many important terms used in this document are defined in Unicode [2]. The terms "left-to-right" (LTR) and "right-to-left" (RTL) are defined in Unicode Standard Annex #9 [3]. The term "formatted text" is defined in RFC 7764 [4].

Block
Any chunk of text that can be parsed unambiguously in one pass. Blocks may contain one or more children which may be other blocks or spans. For example:
  • A single line of text comprising one or more spans
  • A block quotation
  • A preformatted code block
Formal markup language
A structured markup language such as LaTeX, SGML, HTML, or XML that is formally defined and may include metadata unrelated to formatting or text style.
Plain text
Text that does not convey any particular formatting or interpretation of the text by computer programs.
Span
A group of text that may be rendered inline alongside other spans. Spans may be either plain text with no formatting applied, or may be formatted text that is enclosed by two styling directives. Some spans may contain child spans.
Styling directive
A character or set of characters that indicates the beginning of a span or block. For example, in certain contexts the characters '*' (U+002A ASTERISK), and '_' (U+005F LOW LINE) may be styling directives that indicate the beginning of a strong or emphasis span and the string '```' (U+0060 GRAVE ACCENT) may be a styling directive that indicate the beginning of a preformatted code block.
Whitespace character
Any Unicode scalar value which has the property "White_Space" or is in category Z in the Unicode Character Database.

5. Discovering Support

Clients that support message styling MUST advertise the fact by including a feature of "urn::xmpp:styling:0" in response to Service Discovery (XEP-0030) [5] information requests and in their Entity Capabilities (XEP-0115) [6] profiles.

Example 1. Disco info response
<query xmlns='http://jabber.org/protocol/disco#info'>
  …
  <feature var='urn:xmpp:styling:0' />
  …
</query>

6. Business Rules

6.1 Blocks

Parsers implementing message styling will first parse blocks and then parse child blocks or spans if allowed by the specific block type.

6.1.1 Plain

Individual lines of text that are not inside of a preformatted text block are considered a "plain" block. Plain blocks are not bound by styling directives and do not imply formatting themselves, but they may contain spans which imply formatting. Plain blocks may not contain child blocks.

Example 2. Plain block text
<body>
  There are three blocks in this body, one per line,
  but there is no *formatting
  as spans* may not escape blocks.
</body>

6.1.2 Preformatted Text

A preformatted text block is started by a line beginning with "```" (U+0060 GRAVE ACCENT), and ended by a line containing only three grave accents or the end of the parent block (whichever comes first). Preformatted text blocks cannot contain child blocks or spans. Text inside a preformatted block SHOULD be displayed in a monospace font.

Example 3. Preformatted block text
<body>
  ```ignored
  (println &quot;Hello, world!&quot;)
  ```

  This should show up as monospace, preformatted text ⤴
</body>
Example 4. No closing preformatted text sequence
<body>
  &gt; ```
  &gt; (println &quot;Hello, world!&quot;)

  The entire blockquote is a preformatted text block, but this line
  is plaintext!
</body>

6.1.3 Quotations

A quotation is indicated by one or more lines with a byte stream beginning with a '>' (U+003E GREATER-THAN SIGN). They are terminated by the first new line that is not followed by a greater-than sign, or the end of the parent block (whichever comes first). Block quotes may contain any child block, including other quotations. Lines inside the block quote MUST have the first leading whitespace character trimmed before parsing the child block. It is RECOMMENDED that text inside of a block quote be indented or distinguished from the surrounding text in some other way.

Example 5. Quotation (LTR)
<body>
  &gt; That that is, is.

  Said the old hermit of Prague.
</body>
Example 6. Nested Quotation
<body>
  &gt;&gt; That that is, is.
  &gt; Said the old hermit of Prague.

  Who?
</body>

6.2 Spans

Spans are always the children of blocks and may not escape from their containing block. Matches of spans between two styling directives MUST contain some text between the two directives, otherwise neither directive is valid. The opening styling directive MUST be located at the beginning of the parent block, after a whitespace character, or after a different opening styling directive. The opening styling directive MUST NOT be followed by a whitespace character and the closing styling directive MUST NOT be preceeded by a whitespace character. Spans are always parsed from the beginning of the byte stream to the end and are lazily matched. Characters that would be styling directives but do not follow these rules are not considered when matching and thus may be present between two other styling directives.

For example, each of the following would be styled as indicated:

Nothing would be styled in the following messages (where "\n" represents a new line):

6.2.1 Plain

Any text inside of a block that is not part of another span is implicitly considered to be inside of a "plain text" span. In the following example the plain span is everything before the first "*".

Example 7. Plain
<body>
  Two spans, both *alike in dignity*
</body>

6.2.2 Emphasis

Text enclosed by '_' (U+005F LOW LINE) is emphasized and SHOULD be displayed in italics.

Example 8. Italic
<body>
  The full title is _Twelfth Night, or What You Will_ but
  _most_ people shorten it.
</body>

6.2.3 Strong Emphasis

Text enclosed by '*' (U+002A ASTERISK) is strongly emphasized and SHOULD be displayed with a heavier font weight than the surrounding text (bold).

Example 9. Strong
<body>
  The full title is "Twelfth Night, or What You Will" but
  *most* people shorten it.
</body>

6.2.4 Strike through

Text enclosed by '~' (U+007E TILDE) SHOULD be displayed with a horizontal line through the middle.

Example 10. Strike through
<body>
  Everyone ~dis~likes cake.
</body>

6.2.5 Preformatted Span

Text enclosed by a '`' (U+0060 GRAVE ACCENT) is a preformatted span SHOULD be displayed inline in a monospace font. A preformatted span may only contain a single plain span. Inline formatting directives inside the preformatted span are not rendered. For example, the following all contain valid preformatted spans:

Example 11. Monospace text
<body>
  Wow, I can write in `monospace`!
</body>

7. Disabling Styling

On rare occasions styling hints may conflict with the contents of a message. For example, if the user sends the emoji "> _ <" it would be interpreted as a block quote. Senders may indicate to the receiver that a particular message SHOULD NOT be styled by adding an empty <unstyled> element qualified by the "urn:xmpp:styling:0" namespace.

Example 12. Sender indicates that styling is disabled
<message>
  <body>&gt; _ &lt;</body>
  <unstyled xmlns="urn:xmpp:styling:0"/>
</message>

8. Implementation Notes

This document does not define a regular grammar and thus styling cannot be matched by a regular expression. Instead, a simple parser can be constructed by first parsing all text into blocks and then recursively parsing the child-blocks inside block quotations, the spans inside individual lines, and by returning the text inside preformatted blocks without modification.

It is RECOMMENDED that styling directives be displayed and formatted in the same manner as the text they apply to. For example, the string "*emphasis*" would be rendered as "*emphasis*".

This specification does not provide a mechanism for removing styling from individual spans or blocks within a styled message. Implementations are free to implement their own workarounds, for example by inserting Unicode non-printable characters to invalidate styling directives, but no specific technique is known to be widely supported.

9. Accessibility Considerations

When displaying styling directives, developers should ensure sufficient contrast so that visually impaired users are able to distinguish the styling directives from the background color. Care should also be taken to ensure that formatting designed to differentiate styling directives from surrounding text does not make the text more difficult to read for visually impaired users.

Styled text may be rendered poorly by screen readers. When applying formatting it may be desirable to include directives to exclude styling directives from being read, or to add markers to the final output that have semantic meaning for screen readers. For example, in an web based client an emphasis span might be converted to an HTML <em> element.

10. Security Considerations

Authors of message styling parsers should take care that improperly formatted messages cannot lead to buffer overruns or code execution.

Though message styling is not compatible with Markdown, some of its styles are similar. Markdown parsers often allow arbitrary HTML to be inserted into documents and therefore MUST NOT be used to parse the format defined in this document even if they are flexible enough to do so.

11. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [7].

12. XMPP Registrar Considerations

12.1 Protocol Namespaces

This specification defines the following XML namespace:

Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar [8] shall add the foregoing namespace to the registries located at <https://xmpp.org/registrar/disco-features.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) [9].

Service Discovery Features Registry Submission
<var>
  <name>urn:xmpp:styling:0</name>
  <desc>Support for rendering message styling.</desc>
  <doc>&xep0393;</doc>
</var>

The XMPP Registrar [8] shall also add the foregoing namespace to the Jabber/XMPP Protocol Namespaces Registry located at <https://xmpp.org/registrar/namespaces.html>. Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar [8] shall remove the provisional status from this registry entry.

Jabber/XMPP Protocol Namespaces Registry Submission
<ns>
  <name>urn:xmpp:styling:0</name>
  <doc>&xep0393;</doc>
  <status>provisional</status>
</ns>

12.2 Namespace Versioning

If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.

13. XML Schema

This document does not define any new XML structure requiring a schema.

14. Acknowledgements

The author wishes to thank Kevin Smith for his review and feedback.


Appendices

Appendix A: Document Information

Series
XEP
Number
0393
Publisher
XMPP Standards Foundation
Status
Draft
Type
Standards Track
Version
1.1.1
Last Updated
2021-04-04
Approving Body
XMPP Council
Dependencies
XMPP Core, XEP-0001
Supersedes
XEP-0071
Superseded By
None
Short Name
styling
Source Control
HTML

This document in other formats: XML  PDF

Appendix B: Author Information

Sam Whited
Email
sam@samwhited.com
JabberID
sam@samwhited.com
URI
https://blog.samwhited.com/

Copyright

This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Visual Presentation

The HTML representation (you are looking at) is maintained by the XSF. It is based on the YAML CSS Framework, which is licensed under the terms of the CC-BY-SA 2.0 license.

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

1. XEP-0071: XHTML-IM <https://xmpp.org/extensions/xep-0071.html>.

2. The Unicode Standard, The Unicode Consortium <http://www.unicode.org/versions/latest/>.

3. Unicode Standard Annex #9, "Unicode Bidirectional Algorithm", edited by Mark Davis, Aharon Lanin, and Andrew Glass. An integral part of The Unicode Standard, <http://unicode.org/reports/tr9/>.

4. RFC 7764: Guidance on Markdown: Design Philosophies, Stability Strategies, and Select Registrations <http://tools.ietf.org/html/rfc7764>.

5. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.

6. XEP-0115: Entity Capabilities <https://xmpp.org/extensions/xep-0115.html>.

7. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

8. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.

9. XEP-0053: XMPP Registrar Function <https://xmpp.org/extensions/xep-0053.html>.

Appendix H: Revision History

Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/

  1. Version 1.1.1 (2021-04-04)

    General readability changes: Copy part of span definition from glossary to business rules and remove parenthesis for marking spans and blocks which was used inconsistently. Also merge two lists of examples.

    ssw
  2. Version 1.1.0 (2021-01-12)

    Clarify parsing rules for span directives

    sw
  3. Version 1.0.0 (2020-10-13)

    Accepted as Draft as per Council vote from 2020-10-07.

    XEP Editor (jsc)
  4. Version 0.4.0 (2020-06-02)

    Remove description of mechanism for disabling styling on individual spans and blocks, users can do this themselves without us documenting the use of a codepoint that's not specifically for this purpose.

    ssw
  5. Version 0.3.0 (2020-06-02)

    Add ability to disable styling, further clarify accessibility considerations, and mention that Styling is not Markdown in the security considerations section.

    ssw
  6. Version 0.2.2 (2020-05-20)

    Clarify accessibility considerations section and be consistent about use of the term "styling directives".

    ssw
  7. Version 0.2.1 (2020-01-02)

    Make rules consistent with examples that show multiple styling directives being applied to the same text.

    ssw
  8. Version 0.2.0 (2019-09-02)

    Clarify block quote termination and white space trimming.

    ssw
  9. Version 0.1.4 (2018-05-01)

    Clarify language around strong emphasis.

    ssw
  10. Version 0.1.3 (2018-02-14)

    Reorder block and span sections, simplify block parsing, and update the definition of a span.

    ssw
  11. Version 0.1.2 (2018-01-13)

    Clarify block quote and plain text parsing and formatting behavior.

    ssw
  12. Version 0.1.1 (2018-01-12)

    Minor clarifications and updates, add security considerations, and expand the glossary.

    ssw
  13. Version 0.1.0 (2017-11-22)

    First draft approved by the XMPP Council.

    XEP Editor (ssw)
  14. Version 0.0.1 (2017-10-28)

    First draft.

    ssw

Appendix I: Bib(La)TeX Entry

@report{whited2017styling,
  title = {Message Styling},
  author = {Whited, Sam},
  type = {XEP},
  number = {0393},
  version = {1.1.1},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-0393.html},
  date = {2017-10-28/2021-04-04},
}

END