mirror of
https://github.com/moparisthebest/xeps
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613 lines
21 KiB
XML
613 lines
21 KiB
XML
<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Message Styling</title>
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<abstract>
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This specification defines a formatted text syntax for use in instant
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messages with simple text styling.
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</abstract>
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&LEGALNOTICE;
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<number>0393</number>
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<status>Draft</status>
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<lastcall>2020-05-26</lastcall>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>XMPP Core</spec>
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<spec>XEP-0001</spec>
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</dependencies>
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<supersedes><spec>XEP-0071</spec></supersedes>
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<supersededby/>
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<shortname>styling</shortname>
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&sam;
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<revision>
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<version>1.1.0</version>
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<date>2021-01-12</date>
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<initials>sw</initials>
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<remark><p>Clarify parsing rules for span directives</p></remark>
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</revision>
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<revision>
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<version>1.0.0</version>
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<date>2020-10-13</date>
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<initials>XEP Editor (jsc)</initials>
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<remark><p>Accepted as Draft as per Council vote from 2020-10-07.</p></remark>
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</revision>
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<revision>
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<version>0.4.0</version>
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<date>2020-06-02</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Remove description of mechanism for disabling styling on individual
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spans and blocks, users can do this themselves without us documenting
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the use of a codepoint that's not specifically for this purpose.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.3.0</version>
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<date>2020-06-02</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Add ability to disable styling, further clarify accessibility
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considerations, and mention that Styling is not Markdown in the security
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considerations section.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.2.2</version>
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<date>2020-05-20</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Clarify accessibility considerations section and be consistent about use
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of the term "styling directives".
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.2.1</version>
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<date>2020-01-02</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Make rules consistent with examples that show multiple styling
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directives being applied to the same text.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.2.0</version>
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<date>2019-09-02</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Clarify block quote termination and white space trimming.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.1.4</version>
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<date>2018-05-01</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Clarify language around strong emphasis.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.1.3</version>
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<date>2018-02-14</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Reorder block and span sections, simplify block parsing, and update the
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definition of a span.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.1.2</version>
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<date>2018-01-13</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Clarify block quote and plain text parsing and formatting behavior.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.1.1</version>
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<date>2018-01-12</date>
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<initials>ssw</initials>
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<remark>
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<p>
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Minor clarifications and updates, add security considerations, and
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expand the glossary.
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</p>
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</remark>
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</revision>
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<revision>
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<version>0.1.0</version>
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<date>2017-11-22</date>
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<initials>XEP Editor (ssw)</initials>
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<remark><p>First draft approved by the XMPP Council.</p></remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2017-10-28</date>
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<initials>ssw</initials>
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<remark><p>First draft.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>
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Historically, XMPP has had no system for simple text styling.
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Instead, specifications like &xep0071; that require full layout engines have
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been used, leading to numerous security issues with implementations.
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Some entities have also performed their own styling based on identifiers in
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the body.
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While this has worked well in the past, it is not interoperable and leads to
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entities each supporting their own informal styling languages.
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</p>
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<p>
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This specification aims to provide a single, interoperable formatted text
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syntax that can be used by entities that do not require full layout engines.
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</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<ul>
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<li>
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Clients that do not support this specification MUST still be able to
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receive messages sent by clients using this specification and display them
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in a human-readable form.
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</li>
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<li>
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Clients that support this specification MUST NOT be required to use a
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layout engine such as HTML or LaTeX.
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</li>
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<li>
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Messages formatted using this specification MUST NOT hinder readability on
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receiving clients regardless of client background color, contrast, or
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window size.
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</li>
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<li>
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Messages formatted using this specification MUST NOT hinder readability by
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users with color vision deficiency or impaired vision.
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</li>
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<li>
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Messages formatted with this specification MUST render correctly in
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locales with right-to-left (RTL) layouts without causing confusion.
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</li>
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<li>
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Clients that support this specification MUST NOT be required to extract
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metadata unrelated to formatting or text style from the message.
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</li>
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<li>
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Servers MUST NOT need to implement any new functionality for this
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specification to be supported.
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</li>
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</ul>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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<ul>
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<li>
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As a user sending an instant message to a friend, I want to be able to
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emphasize an important part of my message.
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</li>
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<li>
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As a software developer, I want to be able to send code as pre-formatted,
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monospace, block or inline text to another developer.
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</li>
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<li>
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As a multi-user chat user I want to add context to my reply by quoting an
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earlier message in the chat.
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</li>
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</ul>
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</section1>
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<section1 topic='Glossary' anchor='glossary'>
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<p>
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Many important terms used in this document are defined in &unicode;.
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The terms "left-to-right" (LTR) and "right-to-left" (RTL) are defined in
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&uax9;.
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The term "formatted text" is defined in &rfc7764;.
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</p>
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<dl>
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<di>
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<dt>Block</dt>
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<dd>
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Any chunk of text that can be parsed unambiguously in one pass.
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Blocks may contain one or more children which may be other blocks or
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spans.
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For example:
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<ul>
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<li>A single line of text comprising one or more spans</li>
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<li>A block quotation</li>
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<li>A preformatted code block</li>
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</ul>
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</dd>
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</di>
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<di>
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<dt>Formal markup language</dt>
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<dd>
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A structured markup language such as LaTeX, SGML, HTML, or XML that is
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formally defined and may include metadata unrelated to formatting or
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text style.
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</dd>
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</di>
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<di>
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<dt>Plain text</dt>
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<dd>
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Text that does not convey any particular formatting or interpretation of
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the text by computer programs.
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</dd>
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</di>
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<di>
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<dt>Span</dt>
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<dd>
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A group of text that may be rendered inline alongside other spans.
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Spans may be either plain text with no formatting applied, or may be
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formatted text that is enclosed by two styling directives.
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Spans are always children of blocks and may not escape from their
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containing block.
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Some spans may contain child spans.
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The following all contain spans marked by parenthesis:
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<ul>
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<li>(plain span)</li>
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<li>(<strong>*strong span*</strong>)</li>
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<li>(<em>_emphasized span_</em>)</li>
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<li>(<em>_emphasized span containing </em>(<em><strong>*strong span*</strong></em>)<em>_</em>)</li>
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<li>(span one )(<strong>*span two*</strong>)</li>
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</ul>
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</dd>
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</di>
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<di>
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<dt>Styling directive</dt>
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<dd>
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A character or set of characters that indicates the beginning of a span
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or block.
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For example, in certain contexts the characters '*' (U+002A ASTERISK),
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and '_' (U+005F LOW LINE) may be styling directives that indicate the
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beginning of a strong or emphasis span and the string '```' (U+0060
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GRAVE ACCENT) may be a styling directive that indicate the beginning of
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a preformatted code block.
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</dd>
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</di>
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<di>
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<dt>Whitespace character</dt>
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<dd>
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Any Unicode scalar value which has the property "White_Space" or is in
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category Z in the Unicode Character Database.
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</dd>
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</di>
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</dl>
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</section1>
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<section1 topic='Discovering Support' anchor='disco'>
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<p>
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Clients that support message styling MUST advertise the fact by including a
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feature of "urn::xmpp:styling:0" in response to &xep0030; information
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requests and in their &xep0115; profiles.
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</p>
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<example caption='Disco info response'><![CDATA[
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<query xmlns='http://jabber.org/protocol/disco#info'>
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…
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<feature var='urn:xmpp:styling:0' />
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…
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</query>]]></example>
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</section1>
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<section1 topic='Business Rules' anchor='rules'>
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<section2 topic='Blocks' anchor='block'>
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<p>
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Parsers implementing message styling will first parse blocks and then
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parse child blocks or spans if allowed by the specific block type.
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</p>
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<section3 topic='Plain' anchor='line-block'>
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<p>
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Individual lines of text that are not inside of a preformatted text
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block are considered a "plain" block.
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Plain blocks are not bound by styling directives and do not imply
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formatting themselves, but they may contain spans which imply
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formatting.
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Plain blocks may not contain child blocks.
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</p>
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<example caption='Plain block text'><![CDATA[
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<body>
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(There are three blocks in this body marked by parens,)
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(but there is no *formatting)
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(as spans* may not escape blocks.)
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</body>
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]]></example>
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</section3>
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<section3 topic='Preformatted Text' anchor='pre-block'>
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<p>
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A preformatted text block is started by a line beginning with "```"
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(U+0060 GRAVE ACCENT), and ended by a line containing only three grave
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accents or the end of the parent block (whichever comes first).
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Preformatted text blocks cannot contain child blocks or spans.
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Text inside a preformatted block SHOULD be displayed in a monospace font.
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</p>
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<example caption='Preformatted block text'><![CDATA[
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<body>
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```ignored
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(println "Hello, world!")
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```
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This should show up as monospace, preformatted text ⤴
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</body>
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]]></example>
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<example caption='No closing preformatted text sequence'><![CDATA[
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<body>
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> ```
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> (println "Hello, world!")
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The entire blockquote is a preformatted text block, but this line
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is plaintext!
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</body>
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]]></example>
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</section3>
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<section3 topic='Quotations' anchor='quote'>
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<p>
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A quotation is indicated by one or more lines with a byte stream
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beginning with a '>' (U+003E GREATER-THAN SIGN).
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They are terminated by the first new line that is not followed by a
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greater-than sign, or the end of the parent block (whichever comes
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first).
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Block quotes may contain any child block, including other quotations.
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Lines inside the block quote MUST have the first leading whitespace
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character trimmed before parsing the child block.
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It is RECOMMENDED that text inside of a block quote be indented or
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distinguished from the surrounding text in some other way.
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</p>
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<example caption='Quotation (LTR)'><![CDATA[
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<body>
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> That that is, is.
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Said the old hermit of Prague.
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</body>
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]]></example>
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<example caption='Nested Quotation'><![CDATA[
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<body>
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>> That that is, is.
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> Said the old hermit of Prague.
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Who?
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</body>
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]]></example>
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</section3>
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</section2>
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<section2 topic='Spans' anchor='span'>
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<p>
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Matches of spans between two styling directives MUST contain some text
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between the two styling directives, otherwise neither directive is valid.
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The opening styling directive MUST be located at the beginning of the
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line, after a whitespace character, or after a different opening styling
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directive.
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The opening styling directive MUST NOT be followed by a whitespace
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character and the closing styling directive MUST NOT be preceeded by a
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whitespace character.
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Spans are always parsed from the beginning of the byte stream to the end
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and are lazily matched.
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Characters that would be styling directives but do not follow these rules
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are not considered when matching and thus may be present between two other
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styling directives.
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</p>
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<p>
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For example, each of the following would be styled as indicated:
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</p>
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<ul>
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<li><strong>*strong*</strong></li>
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<li>plain <strong>*strong*</strong> plain</li>
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<li><strong>*strong*</strong> plain <strong>*strong*</strong></li>
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<li><strong>*strong*</strong>plain*</li>
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<li>* plain <strong>*strong*</strong></li>
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</ul>
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<p>
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Nothing would be styled in the following messages (where "\n" represents a
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new line):
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</p>
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<ul>
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<li>not strong*</li>
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<li>*not strong</li>
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<li>*not \n strong*</li>
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<li>*not *strong</li>
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<li>**</li>
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<li>***</li>
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<li>****</li>
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</ul>
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<section3 topic='Plain' anchor='plain'>
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<p>
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Any text inside of a block that is not part of another span is
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implicitly considered to be inside of a "plain text" span.
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</p>
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<example caption='Plain'><![CDATA[
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<body>
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(Two spans, both )(*alike in dignity*)
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</body>
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]]></example>
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</section3>
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<section3 topic='Emphasis' anchor='emph'>
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<p>
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Text enclosed by '_' (U+005F LOW LINE) is emphasized and SHOULD be
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displayed in italics.
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</p>
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<example caption='Italic'><![CDATA[
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<body>
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The full title is _Twelfth Night, or What You Will_ but
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_most_ people shorten it.
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</body>
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]]></example>
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</section3>
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<section3 topic='Strong Emphasis' anchor='strong'>
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<p>
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Text enclosed by '*' (U+002A ASTERISK) is strongly emphasized and SHOULD
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be displayed with a heavier font weight than the surrounding text
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(bold).
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</p>
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<example caption='Strong'><![CDATA[
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<body>
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The full title is "Twelfth Night, or What You Will" but
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*most* people shorten it.
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</body>
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]]></example>
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</section3>
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<section3 topic='Strike through' anchor='strike'>
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<p>
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Text enclosed by '~' (U+007E TILDE) SHOULD be displayed with a horizontal
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line through the middle.
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</p>
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<example caption='Strike through'><![CDATA[
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<body>
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Everyone ~dis~likes cake.
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</body>
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]]></example>
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</section3>
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<section3 topic='Preformatted Span' anchor='mono'>
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<p>
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Text enclosed by a '`' (U+0060 GRAVE ACCENT) is a preformatted span SHOULD
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be displayed inline in a monospace font.
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A preformatted span may only contain a single plain span.
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Inline formatting directives inside the preformatted span are not
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rendered.
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For example, the following all contain valid preformatted spans:
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</p>
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<ul>
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<li>This is <tt>`monospace`</tt></li>
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<li>This is <tt>`*monospace*`</tt></li>
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<li>This is <strong><tt>*`monospace and bold`*</tt></strong></li>
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</ul>
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<example caption='Monospace text'><![CDATA[
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<body>
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Wow, I can write in `monospace`!
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</body>
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]]></example>
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</section3>
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</section2>
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</section1>
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<section1 topic='Disabling Styling' anchor='disable'>
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<p>
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On rare occasions styling hints may conflict with the contents of a message.
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For example, if the user sends the emoji "> _ <" it would be
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interpreted as a block quote.
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Senders may indicate to the receiver that a particular message SHOULD NOT be
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styled by adding an empty <unstyled> element qualified by the
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"urn:xmpp:styling:0" namespace.
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</p>
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<example caption='Sender indicates that styling is disabled'><![CDATA[
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<message>
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<body>> _ <</body>
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|
<unstyled xmlns="urn:xmpp:styling:0"/>
|
|
</message>
|
|
]]></example>
|
|
</section1>
|
|
<section1 topic='Implementation Notes' anchor='impl'>
|
|
<p>
|
|
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.
|
|
</p>
|
|
<p>
|
|
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
|
|
"<strong>*emphasis*</strong>".
|
|
</p>
|
|
<p>
|
|
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.
|
|
</p>
|
|
</section1>
|
|
<section1 topic='Accessibility Considerations' anchor='access'>
|
|
<p>
|
|
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.
|
|
</p>
|
|
<p>
|
|
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.
|
|
</p>
|
|
</section1>
|
|
<section1 topic='Security Considerations' anchor='security'>
|
|
<p>
|
|
Authors of message styling parsers should take care that improperly
|
|
formatted messages cannot lead to buffer overruns or code execution.
|
|
</p>
|
|
<p>
|
|
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.
|
|
</p>
|
|
</section1>
|
|
<section1 topic='IANA Considerations' anchor='iana'>
|
|
<p>
|
|
This document requires no interaction with &IANA;.
|
|
</p>
|
|
</section1>
|
|
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
|
|
<section2 topic='Protocol Namespaces' anchor='registrar-ns'>
|
|
<p>This specification defines the following XML namespace:</p>
|
|
<ul>
|
|
<li>urn:xmpp:styling:0</li>
|
|
</ul>
|
|
<p>
|
|
Upon advancement of this specification from a status of Experimental to a
|
|
status of Draft, the ®ISTRAR; shall add the foregoing namespace to the
|
|
registries located at &DISCOFEATURES;, as described in Section 4 of
|
|
&xep0053;.
|
|
</p>
|
|
<code caption='Service Discovery Features Registry Submission'><![CDATA[
|
|
<var>
|
|
<name>urn:xmpp:styling:0</name>
|
|
<desc>Support for rendering message styling.</desc>
|
|
<doc>&xep0393;</doc>
|
|
</var>]]></code>
|
|
<p>
|
|
The ®ISTRAR; shall also add the foregoing namespace to the Jabber/XMPP
|
|
Protocol Namespaces Registry located at &NAMESPACES;.
|
|
Upon advancement of this specification from a status of Experimental to a
|
|
status of Draft, the ®ISTRAR; shall remove the provisional status from
|
|
this registry entry.
|
|
</p>
|
|
<code caption='Jabber/XMPP Protocol Namespaces Registry Submission'><![CDATA[
|
|
<ns>
|
|
<name>urn:xmpp:styling:0</name>
|
|
<doc>&xep0393;</doc>
|
|
<status>provisional</status>
|
|
</ns>]]></code>
|
|
</section2>
|
|
<section2 topic='Namespace Versioning' anchor='registrar-versioning'>
|
|
&NSVER;
|
|
</section2>
|
|
</section1>
|
|
<section1 topic='XML Schema' anchor='schema'>
|
|
<p>This document does not define any new XML structure requiring a schema.</p>
|
|
</section1>
|
|
<section1 topic='Acknowledgements' anchor='ack'>
|
|
<p>The author wishes to thank Kevin Smith for his review and feedback.</p>
|
|
</section1>
|
|
</xep>
|