This document provides a checklist of internationalization-related considerations when developing a specification. Most checklist items point to detailed supporting information in other documents. Where such information does not yet exist, it can be given a temporary home in this document. The dynamic page Internationalization Techniques: Developing specifications is automatically generated from this document. The current version is still an early draft, and it is expected that the information will change regularly as new content is added and existing content is modified in the light of experience and discussion.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document provides advice to specification developers about how to incorporate requirements for international use. What is currently available here is expected to be useful immediately, but is still an early draft and the document is in flux, and will grow over time as knowledge applied in reviews and discussions can be crystallized into guidelines.


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1. Introduction

Developers of specifications need advice to ensure that what they produce will work for communities around the globe.

The Internationalization (i18n) WG tries to assist working groups by reviewing specifications and engaging in discussion. Often, however, such interventions come later in the process than would be ideal, or mean that the i18n WG has to repeat the same information for each working group it interacts with.

It would be better if specification developers could access a checklist of best practices, which points to explanations, examples and rationales where developers need it. Developers would then be able to build this knowledge into their work from the earliest stages, and could thereby reduce rework needed when the i18n WG reviews their specification.

This document contains the beginnings of a checklist, and points to locations where you can find explanations, examples and rationales for recommendations made.  If there is no such other place, that extra information will be added to this document. It is still early days for this document, and it may also be used to develop ideas and organize them.

You may prefer to use Internationalization Techniques: Developing specifications most of the time, since it uses JavaScript to help you more quickly see what's available and drill down to the information you need. (Where needed, it links to this or other documents.) There is also a non-dynamic version of the document available.

2. Language

2.1 Language basics

It should be possible to associate a language with any piece of natural language text that will be read by a user. more

Where possible, there should be a way to label natural language changes in inline text. more

Text is rendered or processed differently according to the language it is in. For example, screen readers need to be prompted when a language changes, and spell checkers should be language-sensitive. When rendering text a knowledge of language is need in order to apply correct fonts, hyphenation, line-breaking, upper/lower case changes, and other features.

For example, ideographic characters such as 雪, 刃, 直, 令, 垔 have slight but important differences when used with Japanese vs Chinese fonts, and it's important not to apply a Chinese font to the Japanese text, and vice versa when it is presented to a user.

Consider whether it is useful to express the intended linguistic audience of a resource, in addition to specifying the language used for text processing. more

Language information for a given resource can be used with two main objectives in mind: for text-processing, or as a statement of the intended use of the resource. We will explain the difference below.

2.1.1 Text-processing language information

When specifying the text-processing language you are declaring the language in which a specific range of text is actually written, so that user agents or applications that manipulate the text, such as voice browsers, spell checkers, style processors, hyphenators, etc., can apply the appropriate rules to the text in question. So we are, by necessity, talking about associating a single language with a specific range of text.

A language declaration that indicates the text-processing language for a range of text must associate a single language value with a specific range of text. more

It is normal to express a text-processing language as the default language, for processing the resource as a whole, but it may also be necessary to indicate where the language changes within the resource.

Use the HTML lang and XML xml:lang language attributes where appropriate, rather than creating a new attribute or mechanism. more

To identify the text-processing language for a range of text, HTML provides the lang attribute, while XML provides xml:lang which can be used in all XML formats. It's useful to continue using those attributes for relevant markup formats, since authors recognize them, as do HTML and XML processors.

2.1.2 Language metadata about the resource as a whole

It may also be useful to describe the language of a resource as a whole. This type of language declaration typically indicates the intended use of the resource. For example, such metadata may be used for searching, serving the right language version, classification, etc.

This type of language declaration differs from that of the text-processing declaration in that (a) the value for such declarations may be more than one language subtag, and (b) the language value declared doesn't indicate which bits of a multilingual resource are in which language.

The language(s) describing the intended use of a resource do not necessarily include every language used in a document. For example, many documents on the Web contain embedded fragments of content in different languages, whereas the page is clearly aimed at speakers of one particular language. For example, a German city-guide for Beijing may contain useful phrases in Chinese, but it is aimed at a German-speaking audience, not a Chinese one.

On the other hand, it is also possible to imagine a situation where a document contains the same or parallel content in more than one language. For example, a web page may welcome Canadian readers with French content in the left column, and the same content in English in the right-hand column. Here the document is equally targeted at speakers of both languages, so there are two audience languages. Another use case is a blog or a news page aimed at a multilingual community, where some articles on a page are in one language and some in another. In this case, it may make sense to list more than one language tag as the value of the language declaration.

A metadata-type language declaration that indicates the intended use of the resource, rather than the language of a specific range of text, may be associated with multiple language values. more

2.2 Defining language values

Values for language declarations must use BCP 47. more

BCP 47 defines a method to combine subtags in order to create a much more powerful notation for language tags than that provided by the old ISO lists, but it is also backwards compatible with the ISO lists.

For an overview of the key features of BCP 47, see Language tags in HTML and XML.

Refer to BCP 47, not to RFC 5646. more

The link to and name of BCP 47 was created specifically so that there is an unchanging reference to the definition of Tags for the Identification of Languages. RFCs 3066, 4646 and 5646 are versions of BCP 47.

Be specific about what level of conformance you expect for language tags. The word "valid" has special meaning in BCP 47. Generally "well-formed" is a better choice.

A well-formed BCP 47 language tag has hyphen-separated subtags with specific lengths and in a particular order. Valid BCP 47 language tags are well-formed but also use only subtags that are listed in the IANA Subtag Registry. Note that the IANA Subtag Registry is frequently updated with new subtags.

Reference BCP47 for language tag matching.

BCP 47 contains one RFC dedicated to the syntax and subtags of language tags, and another dedicated to how to match two or more subtags. (This topic needs more detail, and may merit being a separate section.)

2.3 Declaring language at the resource level

Here we are talking about an independent unit of data that contains structured text. Examples may include a whole HTML page, an XML document, a JSON file, a WebVTT script, an annotation, etc.

The specification should indicate how to define the default text-processing language for the resource as a whole. more

It often saves trouble to identify the language, or at least the default language, of the resource as a whole in one place. For example, in an HTML file, this is done by setting the lang attribute on the html element.

Content within the resource should inherit the language of the text-processing declared at the resource level, unless it is specifically overridden.

Consider whether it is necessary to have separate declarations to indicate the text-processing language versus metadata about the expected use of the resource. more

In many cases a resource contains text in only one language, and in many more cases the language declared as the default language for text-processing is the same as the language that describes the metadata about the resource as a whole. In such cases it makes sense to have a single declaration.

It becomes problematic, however, to use a single declaration when it refers to more than one language unless there is a way to determine which one language should be used as the text-processing default.

If there is only one language declaration for a resource, and it has more than one language tag as a value, it must be possible to identify the default text-processing language for the resource. more

2.3.2 See also

Defining language values.

2.4 Establishing the language of a content block

The words block and/or chunk are used here to refer to a structural component within the resource as a whole that groups content together and separates it from adjacent content. Boundaries between one block and another are equivalent to paragraph or section boundaries in text, or discrete data items inside a file.

For example, this could refer to a block or paragraph in XML or HTML, an object declaration in JSON, a cue in WebVTT, a line in a CSV file, etc. Contrast this with inline content, which describes a range within a paragraph, sentence, etc.

The interpretation of which structures defined in a spec are relevant to these requirements may require a little consideration, and will depend on the format of the data involved.

By default, blocks of content should inherit any text-processing language set for the resource as a whole. more

See 2.1 Language basics for guidance related to the default text-processing language information.

It should be possible to indicate a change in language for blocks of content where the language changes. more

2.4.2 See also

Defining language values.

2.5 Establishing the language of inline runs

Here we refer to information that needs to be provided for a range of characters in the middle of a paragraph or string.

It should be possible to indicate language for spans of inline text where the language changes. more

Where a switch in language can affect operations on the content, such as spell-checking, rendering, styling, voice production, translation, information retrieval, and so forth, it is necessary to indicate the range of text affected and identify the language of that content.

2.5.2 See also

Defining language values.

3. Text direction

It is important to establish direction for text written or mixed with right-to-left scripts. Characters in these scripts are stored in memory in the order they are typed and pronounced – called the logical order. The Unicode Bidirectional Algorithm (UBA) provides a lot of support for automatically rendering a sequence of characters stored in logical order so that they are visually ordered as expected. Unfortunately, the UBA alone is not sufficient to correctly render bidirectional text, and additional information has to be provided about the default directional context to apply for a given sequence of characters.

3.1 Basic requirements

The basic requirements are as follows.

It must be possible to indicate base direction for each individual paragraph-level item of natural language text that will be read by someone. more

It must be possible to indicate base direction changes for embedded runs of inline bidirectional text for all natural language text that will be read by someone. more

Annotating right-to-left text must require the minimum amount of effort for people who work natively with right-to-left scripts. more

Requiring a speaker of Arabic, Divehi, Hebrew, Persian, Urdu, etc. to add markup or control characters to every paragraph or small data item they write is far too much to be manageable. Typically, the format should establish a default direction and require the user to intervene only when exceptions have to be dealt with.

3.2 Background information

In this section we try to set out some key concepts associated with text direction, so that it will be easier to understand the recommendations that follow.

3.2.1 Important definitions

In order to correctly display text written in a 'right-to-left' script or left-to-right text containing bidirectional elements, it is important to establish the base direction that will be used to dictate the order in which elements of the text will be displayed.

If you are not familiar with what the Unicode Bidirectional Algorithm (UBA) does and doesn't do, and why the base direction is so important, read Unicode Bidirectional Algorithm basics.

In this section, the word paragraph indicates a run of text followed by a hard line-break in plain text, but may signify different things in other situations. In CSV it equates to 'cell', so a single line of comma-separated items is actually a set of comma-separated paragraphs.  In HTML it equates to the lowest level of block element, which is often a p element, but may be things such as div, li, etc., if they only contain text and/or inline elements. In JSON, it often equates to a quoted string value, but if a string value uses markup then paragraphs are associated with block elements, and if the string value is multiple lines of plain text then each line is a paragraph.

The term metadata is used here to mean information which could be an annotation or property associated with the data, or could be markup in scenarios that allow that, or could be a higher-level protocol, etc.

3.2.2 Ways base direction can be set for paragraphs

There are a number of possible ways of setting the base direction.

  1. The base direction of a paragraph may be set by an application or a user applying metadata to the paragraph. Typical values for base direction may include ltr, rtl or auto.
    • The metadata may specifically indicate that heuristics should be used. Then you would expect to consider the actual characters used in order to determine the base direction. (This is what happens if you set dir=auto on an HTML element.)
    • The application may expect metadata, but there may be no such information provided. In this case you would usually expect there to be a default direction specified, and the base direction for a cell would be set to that default. The default is usually LTR. (This is what happens if you have no dir attributes in your HTML file.)
    • Where a format contains many paragraphs or chunks of information, and the language of text in all those chunks is the same, it is sometimes useful to allow a default base direction to be set for and inherited by all. This is what happens when you set the dir attribute on the html tag in HTML. Another example would be a subtitling file containing many cues, all written in Arabic; it would be best to allow the author to say at the start of the file that the default is RTL for all cue text. There should always be a way to override the direction information for a specific paragraph where needed.
  2. If the application expects no metadata to be available it should use heuristics to determine the base direction for each paragraph/cell. A typical solution, and one described by UAX 9 Unicode Bidirectional Algorithm, is to look for the first-strong character in the paragraph/cell. (This is likely to apply if you are looking at plain text that is not expected to be associated with metadata. It only happens with HTML if the direction is set to auto, since HTML specifies a default direction.)
    • Not all paragraphs using the first-strong method will have the correct base direction applied. In some cases, an Arabic or Hebrew, etc, paragraph may start with strong LTR characters. There must be a way to deal with this.
    • Where a syntactic unit contains multiple lines of plain text (for example, a multiline cue text in a subtitling file), the first-strong heuristic needs to be applied to each line separately.
    • There may be special rules that involve ignoring some sequence of characters or type of markup at the start of the paragraph before identifying the first strong character.
    • In some cases there are no strong characters in a paragraph, and the base direction can be critically important for the data to be understood correctly, eg. telephone numbers or MAC addresses. There needs to be a way to resort to an appropriate default for these cases.
  3. Whether or not any metadata is specified, if a paragraph contains a string that starts with one of the Unicode bidi control characters RLI, LRI, FSI, LRE, RLE, LRO, or RLO and ends with PDF/PDI, these characters will determine the base direction for the contained string. These characters, when placed in the content, explicitly override any previously set direction by creating an inline range and assigning a base direction to it.
    • The effect of such characters does not extend past paragraph boundaries, but the range ought to be explicitly ended using the PDF/PDI control character, especially if a paragraph end is not easily detectable by the application.)
    • Because isolation is needed for bidirectional text to work properly, the Unicode Standard says that the isolating control codes RLI, LRI and FSI should be used rather than LRE or RLE. Unfortunately, those characters are still not widely supported.
    • For structural components in markup, above the paragraph level, it is not possible to use the Unicode bidi control characters to define direction for paragraphs, since these are inline controls only, and the effect is terminated by a paragraph end.

When capturing text input by a user it is usually necessary to understand the context in which the user was inputting the data to determine the base direction of the input. In HTML, for example, this may be set by the direction inherited from the html tag, or by the user pressing keys to set the base direction for a form field. It is then necessary to find some way of storing the information about base direction or associating it with the string when rendered. Typically, in this situation, any direction changes internal to the string being input are handled by the user and will be captured as part of the string.

3.2.3 Inline changes to base direction

Embedded ranges of text within a single paragraph may need to have a different base direction. For example,


where the span within the single quotes is in Hebrew/Arabic/Divehi, etc., and needs to have a RTL base direction, instead of the LTR base direction of the surrounding paragraph, in order to place the exclamation mark correctly.

If markup is available to the content author, it is likely to be easier and safer to use markup to indicate such inline ranges (see below). In HTML you would usually use an inline element with a dir attribute to establish the base direction for such runs of text. If you can't mark up the text, such as in HTML's title element, or any environment that handles only plain text content, you have to resort to Unicode's paired control characters to establish the base direction for such an internal range.

Furthermore, inline ranges where the base direction is changed should be isolated from surrounding text, so that the UBA doesn't produce incorrect results due to interference across boundaries. See an example of how this can produce incorrect ordering of things such as text followed by numbers in HTML, or another example of how it can affect lists.

This means that if a content author is using Unicode control codes they should use RLI/LRI...PDI rather than RLE/LRE...PDF.  These isolating codes are fairly new, and applications may not yet support them.

3.2.4 Problems with control characters

Reasons to avoid relying on control characters to set direction include the following:

  1. They are invisible in most editors and are therefore difficult to work with, and can easily lead to orphans and overlapping ranges. They can be particularly difficult to manage when editing bidirectional inline text because it's hard to position the cursor in the correct place. If you ask someone who writes in a right-to-left script, you are likely to find that they dislike using control codes.
  2. Users often don't have the necessary characters available on their keyboard, or have difficulty inputting them.
  3. It is sometimes necessary to choose which to use based on context or the type of the data, and this means that a content author typically needs to select the control codes – specifying control codes in this way for all paragraphs is time-consuming and error-prone.
  4. Processors that extract parts of the data, add to it, or reuse in combination with other text may incorrectly handle the control codes.
  5. Search and comparison algorithms should ignore these characters, but typically don't.

The last two items above may also hold for markup, but implementers often support included markup better than included control codes.

Don't expect users to add control codes at the start and end of every paragraph. That's far too much work.

3.2.5 RLM and LRM

A word about the Unicode characters U+200F RIGHT-TO-LEFT MARK (RLM) and U+200E LEFT-TO-RIGHT MARK (LRM) is warranted at this point.

The first point to be clear about is that neither RLM nor LRM establish the base direction for a range of text.  They are simply invisible characters with strong directional properties.

This means that you cannot use RLM for example, to make the text W3C appear to the left of the Hebrew text in the following example.

The title is "פעילות הבינאום, W3C".

For this you can only use metadata or the paired control characters.

Of course, if you are detecting base direction using first-strong heuristics then RLM and LRM can be useful for setting the base direction where the text in question begins with something that would otherwise give the wrong result, eg.

"نشاط التدويل" is how you say "i18n Activity" in Arabic.

Here an LRM could be placed at the start of the text, before the strong RTL Arabic characters, to prevent the algorithm from assuming that the text should be right-to-left. (Remember that if metadata is used to set the base direction, that character is ignored, unless the metadata specifically says that first-strong heuristics should be used.)

3.2.6 Base direction and language

Do not assume that direction can be determined from language information. more

The following are all reasons you cannot use language tags to provide information about base direction:

  1. you can't produce the auto value with language tags.
  2. some languages are written with both RTL and LTR scripts.
  3. the only reliable part of the language tag that would indicate the base direction is the script tag, but BCP47 recommends that you suppress the use of the script tag for languages that don't usually need it, such as Hebrew (suppressscript: Hebr). Languages, such as Persian, that are usually written in a RTL script may be written in transcribed form, and it's not possible to guarantee that the necessary script tag would be present to carry the directional information. In summary, you won't be able to rely on people supplying script tags as part of the language information in order to influence direction.
  4. the incidence of use of language tags and base direction markers often don't coincide.
  5. they are not semantically equivalent.

3.3 Handling direction in markup

3.3.1 Setting the default base direction

The spec should indicate how to define a default base direction for the resource as a whole, ie. set the overall base direction. more

The default base direction, in the absence of other information, should be LTR. more

3.3.2 Base direction values

Values for the default base direction should include left-to-right, right-to-left, and auto. more

The auto value allows automatic detection of the base direction for a piece of text. For example, the auto value of dir in HTML looks for the first strong directional character in the text, but ignores certain items of markup also, to guess the base direction of the text. Note that automatic detection algorithms are far from perfect. First-strong detection is unable to correctly identify text that is really right-to-left, but that begins with a strong LTR character. Algorithms that attempt to judge the base direction based on contents of the text are also problematic. The best scenario is one where the base direction is known and declared.

3.3.3 Establishing the base direction for paragraphs

The content author must be able to indicate parts of the text where the base direction changes. At the block level, this should be achieved using attributes or metadata, and should not rely on Unicode control characters.

Relying on Unicode control characters to establish direction for every block is not feasible because line breaks terminate the effect of such control characters. It also makes the data much less stable, and unnecessarily difficult to manage if control characters have to appear at every point where they would be needed.

It must be possible to also set the direction for content fragments to auto. This means that the base direction will be determined by examining the content itself.

A typical approach here would be to set the direction based on the first strong directional character outside of any markup, but this is not the only possible method. The algorithm used to determine directionality when direction is set to auto should match that expected by the receiver.

The first-strong algorithm looks for the first character in the paragraph with a strong directional property according to the Unicode definitions. It then sets the base direction of the paragraph according to the direction of that character.

Note that the first-strong algorithm may incorrectly guess the direction of the paragraph when the first character is not typical of the rest of the paragraph, such as when a RTL paragraph or line starts with a LTR brand name or technical term.

For additional information about algorithms for detecting direction, see Estimation algorithms in the document where this was discussed with reference to HTML.

If the overall base direction is set to auto for plain text, the direction of content paragraphs should be determined on a paragraph by paragraph basis.

To indicate the sides of a block of text where relative to the start and end of its contained lines, you should use 'before' and 'after' (maybe block-start/block-end – the terminology is changing), rather than 'top' and 'bottom'.

To indicate the start/end of a line you should use 'start' and 'end' rather than 'left' and 'right'.

Provide dedicated attributes for control of base direction and bidirectional overrides; do not rely on the user applying style properties to arbitrary markup to achieve bidi control.

For example, HTML has a dir attribute that is capable of managing base direction without assistance from CSS styling. XML formats should define dedicated markup to represent directional information, even if they need CSS to achieve the required display, since the text may be used in other ways.

Style sheets such as CSS may not always be used with the data, or carried with the data when it is syndicated, etc. Directional information is fundamentally important to correct display of the data, and should be associated more closely and more permanently with the markup or data.

3.3.4 Setting base direction for inline text

It must be possible to indicate spans of inline text where the base direction changes. If markup is available, this is the preferred method. Otherwise your specification must require that Unicode control characters are recognized by the receiving application, and correctly implemented.

It must be possible to also set the direction for a span to auto. This means that the base direction will be determined by examining the content itself. A typical approach here would be to set the direction based on the first strong directional character outside of any markup. more

The first-strong algorithm looks for the first character in the paragraph with a strong directional property according to the Unicode definitions. It then sets the base direction of the paragraph according to the direction of that character.

Note that the first-strong algorithm may incorrectly guess the direction of the paragraph when the first character is not typical of the rest of the paragraph, such as when a RTL paragraph or line starts with a LTR brand name or technical term.

For additional information about algorithms for detecting direction, see Estimation algorithms in the document where this was discussed with reference to HTML.

If users use Unicode bidirectional control characters, the RLI/LRI/FSI with PDI characters must be supported by the application and recommended (rather than RLE/LRE with PDF) by the spec.

Use of RLM/LRM should be appropriate, and expectations of what those controls can and cannot do should be clear in the spec. more

The Unicode bidirectional control characters U+200F RIGHT-TO-LEFT MARK and U+200E LEFT-TO-RIGHT MARK are not sufficient on their own to manage bidirectional text. They cannot produce a different base direction for embedded text. For that you need to be able to indicate the start and end of the range of the embedded text.  This is best done by markup, if available, or failing that using the other Unicode bidirectional controls mentioned just above.

Provide dedicated attributes for control of base direction and bidirectional overrides; do not rely on the user applying style properties to arbitrary markup to achieve bidi control.

Allow bidi attributes on all inline elements in markup that contain text.

Provide attributes that allow the user to (a) create an embedded base direction or (b) override the bidirectional algorithm altogether; the attribute should allow the user to set the direction to LTR or RTL in either of these two scenarios.

4. Characters

See the Character Model for the World Wide Web: Fundamentals for basic guidelines related to the use of characters and encodings.

See the Encoding specification for further guidelines related to use of character encodings.

Another Character Model document is currently in development, entitled String Matching and Searching. It looks at issues that arise when you try to compare two strings, be it identifiers or authored content.

4.1 Choosing a definition of 'character'

Specifications, software and content MUST NOT require or depend on a one-to-one correspondence between characters and the sounds of a language. more

Specifications, software and content MUST NOT require or depend on a one-to-one mapping between characters and units of displayed text. more

Protocols, data formats and APIs MUST store, interchange or process text data in logical order. more

Independent of whether some implementation uses logical selection or visual selection, characters selected MUST be kept in logical order in storage. more

Specifications of protocols and APIs that involve selection of ranges SHOULD provide for discontiguous logical selections, at least to the extent necessary to support implementation of visual selection on screen on top of those protocols and APIs. more

Specifications and software MUST NOT require nor depend on a single keystroke resulting in a single character, nor that a single character be input with a single keystroke (even with modifiers), nor that keyboards are the same all over the world. more

Specifications, software and content MUST NOT require or depend on a one-to-one relationship between characters and units of physical storage. more

When specifications use the term 'character' the specifications MUST define which meaning they intend. more

Specifications SHOULD use specific terms, when available, instead of the general term 'character'. more

4.1.2 See also

Defining 'string'.

4.2 Defining a Reference Processing Model

Textual data objects defined by protocol or format specifications MUST be in a single character encoding. more

All specifications that involve processing of text MUST specify the processing of text according to the Reference Processing Model described by the rest of the recommendations in this list. more

Specifications MUST define text in terms of Unicode characters, not bytes or glyphs. more

For their textual data objects specifications MAY allow use of any character encoding which can be transcoded to a Unicode encoding form. more

Specifications MAY choose to disallow or deprecate some character encodings and to make others mandatory. Independent of the actual character encoding, the specified behavior MUST be the same as if the processing happened as follows: (a) The character encoding of any textual data object received by the application implementing the specification MUST be determined and the data object MUST be interpreted as a sequence of Unicode characters - this MUST be equivalent to transcoding the data object to some Unicode encoding form, adjusting any character encoding label if necessary, and receiving it in that Unicode encoding form, (b) All processing MUST take place on this sequence of Unicode characters, (c) If text is output by the application, the sequence of Unicode characters MUST be encoded using a character encoding chosen among those allowed by the specification. more

If a specification is such that multiple textual data objects are involved (such as an XML document referring to external parsed entities), it MAY choose to allow these data objects to be in different character encodings. In all cases, the Reference Processing Model MUST be applied to all textual data objects. more

4.2.2 See also

Including and excluding character ranges.

4.3 Including and excluding character ranges

Specifications SHOULD NOT arbitrarily exclude code points from the full range of Unicode code points from U+0000 to U+10FFFF inclusive. more

Specifications MUST NOT allow code points above U+10FFFF. more

Specifications SHOULD NOT allow the use of codepoints reserved by Unicode for internal use. more

Specifications MUST NOT allow the use of surrogate code points. more

Specifications SHOULD exclude compatibility characters in the syntactic elements (markup, delimiters, identifiers) of the formats they define. more

4.3.2 See also

Using the Private Use Area.

4.4 Using the Private Use Area

Specifications MUST NOT require the use of private use area characters with particular assignments. more

Specifications MUST NOT require the use of mechanisms for defining agreements of private use code points. more

Specifications and implementations SHOULD NOT disallow the use of private use code points by private agreement. more

Specifications MAY define markup to allow the transmission of symbols not in Unicode or to identify specific variants of Unicode characters. more

Specifications SHOULD allow the inclusion of or reference to pictures and graphics where appropriate, to eliminate the need to (mis)use character-oriented mechanisms for pictures or graphics. more

4.4.2 See also

Including and excluding character ranges.

4.5 Choosing character encodings

Specifications MUST either specify a unique character encoding, or provide character encoding identification mechanisms such that the encoding of text can be reliably identified. more

When designing a new protocol, format or API, specifications SHOULD require a unique character encoding. more

When basing a protocol, format, or API on a protocol, format, or API that already has rules for character encoding, specifications SHOULD use rather than change these rules. more

When a unique character encoding is required, the character encoding MUST be UTF-8, UTF-16 or UTF-32. more

This guideline needs further consideration: utf-16 and utf-32 are not recommended these days. UTF-8 is the recommended encoding.

Specifications SHOULD avoid using the terms 'character set' and 'charset' to refer to a character encoding, except when the latter is used to refer to the MIME charset parameter or its IANA-registered values. The term 'character encoding', or in specific cases the terms 'character encoding form' or 'character encoding scheme', are RECOMMENDED. more

If the unique encoding approach is not taken, specifications SHOULD require the use of the IANA charset registry names, and in particular the names identified in the registry as 'MIME preferred names', to designate character encodings in protocols, data formats and APIs. more

This guideline needs further consideration: the list of character encodings recommended for Web specifications is listed in the Encoding specification.

Character encodings that are not in the IANA registry SHOULD NOT be used, except by private agreement. more

If an unregistered character encoding is used, the convention of using 'x-' at the beginning of the name MUST be followed. more

If the unique encoding approach is not chosen, specifications MUST designate at least one of the UTF-8 and UTF-16 encoding forms of Unicode as admissible character encodings and SHOULD choose at least one of UTF-8 or UTF-16 as required encoding forms (encoding forms that MUST be supported by implementations of the specification). more

Specifications that require a default encoding MUST define either UTF-8 or UTF-16 as the default, or both if they define suitable means of distinguishing them. more

4.6 Identifying character encodings

Specifications MUST NOT propose the use of heuristics to determine the encoding of data. more

Specifications MUST define conflict-resolution mechanisms (e.g. priorities) for cases where there is multiple or conflicting information about character encoding. more

4.7 Designing character escapes

Specifications should provide a mechanism for escaping characters, particularly those which are invisible or ambiguous. more

It is generally recommended that character escapes be provided so that difficult to enter or edit sequences can be introduced using a plain text editor. Escape sequences are particularly useful for invisible or ambiguous Unicode characters, including zero-width spaces, soft-hyphens, various bidi controls, mongolian vowel separators, etc.

For advice on use of escapes in markup, but which is mostly generalisable to other formats, see Using character escapes in markup and CSS.

Specifications SHOULD NOT invent a new escaping mechanism if an appropriate one already exists. more

The number of different ways to escape a character SHOULD be minimized (ideally to one). more

Escape syntax SHOULD require either explicit end delimiters or a fixed number of characters in each character escape. Escape syntaxes where the end is determined by any character outside the set of characters admissible in the character escape itself SHOULD be avoided. more

Whenever specifications define character escapes that allow the representation of characters using a number, the number MUST represent the Unicode code point of the character and SHOULD be in hexadecimal notation. more

Escaped characters SHOULD be acceptable wherever their unescaped forms are; this does not preclude that syntax-significant characters, when escaped, lose their significance in the syntax. In particular, if a character is acceptable in identifiers and comments, then its escaped form should also be acceptable. more

4.8 Storing text

Protocols, data formats and APIs MUST store, interchange or process text data in logical order. more

Specifications of protocols and APIs that involve selection of ranges SHOULD provide for discontiguous logical selections, at least to the extent necessary to support implementation of visual selection on screen on top of those protocols and APIs. more

4.9 Specifying sort and search functionality

Software that sorts or searches text for users SHOULD do so on the basis of appropriate collation units and ordering rules for the relevant language and/or application. more

Where searching or sorting is done dynamically, particularly in a multilingual environment, the 'relevant language' SHOULD be determined to be that of the current user, and may thus differ from user to user. more

Software that allows users to sort or search text SHOULD allow the user to select alternative rules for collation units and ordering. more

Specifications and implementations of sorting and searching algorithms SHOULD accommodate text that contains any character in Unicode. more

4.10 Converting to a Common Unicode Form

Specifications of text-based formats and protocols MAY specify that all or part of the textual content of that format or protocol is normalized using Unicode Normalization Form C (NFC). more

Specifications that do not normalize MUST document or provide a health-warning if canonically equivalent but disjoint Unicode character sequences represent a security issue. more

Specifications and implementations MUST NOT assume that content is in any particular normalization form. more

Specifications MUST specify that string matching takes the form of "code point-by-code point" comparison of the Unicode character sequence, or, if a specific Unicode character encoding is specified, code unit-by-code unit comparison of the sequences. more

Specifications that define a regular expression syntax MUST provide at least Basic Unicode Level 1 support per Unicode Technical Standard #18: Unicode Regular Expressions and SHOULD provide Extended or Tailored (Levels 2 and 3) support. more

Specifications of text-based formats and protocols that, as part of their syntax definition, require that the text be in normalized form MUST define string matching in terms of normalized string comparison and MUST define the normalized form to be NFC. more

A normalizing text-processing component which receives suspect text MUST NOT perform any normalization-sensitive operations unless it has first either confirmed through inspection that the text is in normalized form or it has re-normalized the text itself. Private agreements MAY, however, be created within private systems which are not subject to these rules, but any externally observable results MUST be the same as if the rules had been obeyed. more

Specifications of text-based languages and protocols SHOULD define precisely the construct boundaries necessary to obtain a complete definition of full-normalization. These definitions SHOULD include at least the boundaries between markup and character data as well as entity boundaries (if the language has any include mechanism) , SHOULD include any other boundary that may create denormalization when instances of the language are processed, but SHOULD NOT include character escapes designed to express arbitrary characters. more

Where operations can produce denormalized output from normalized text input, specifications of API components (functions/methods) that implement these operations MUST define whether normalization is the responsibility of the caller or the callee. Specifications MAY state that performing normalization is optional for some API components; in this case the default SHOULD be that normalization is performed, and an explicit option SHOULD be used to switch normalization off. Specifications SHOULD NOT make the implementation of normalization optional. more

Specifications that define a mechanism (for example an API or a defining language) for producing textual data object SHOULD require that the final output of this mechanism be normalized. more

4.11 Handling Case Folding

Case sensitive matching is RECOMMENDED as the default for new protocols and formats. more

Because the "simple" case-fold mapping removes information that can be important to forming an identity match, the "Common plus Full" (or "Unicode C+F") case fold mapping is RECOMMENDED for Unicode case-insensitive matching. more

ASCII case-insensitive matching MUST only be applied to vocabularies that are restricted to ASCII. Unicode case-insensitivity MUST be used for all other vocabularies. more

If the vocabulary is not restricted to ASCII or permits user-defined values that use a broader range of Unicode, ASCII case-insensitive matching MUST NOT be required. more

The Unicode C+F case-fold form is RECOMMENDED as the case-insensitive matching for vocabularies. The Unicode C+S form MUST NOT be used for string identity matching on the Web. more

Specifications and implementations that define string matching as part of the definition of a format, protocol, or formal language (which might include operations such as parsing, matching, tokenizing, etc.) MUST define the criteria and matching forms used. These MUST be one of: (a) Case-sensitive (b) Unicode case-insensitive using Unicode case-folding C+F (c) ASCII case-insensitive. more

Specifications SHOULD NOT specify case-insensitive comparison of strings. more

Specifications that specify case-insensitive comparison for non-ASCII vocabularies SHOULD specify Unicode case-folding C+F. more

Specifications MAY specify ASCII case-insensitive comparison for portions of a format or protocol that are restricted to an ASCII-only vocabulary. more

Specifications and implementations MUST NOT specify ASCII-only case-insensitive matching for values or constructs that permit non-ASCII characters. more

4.12 Defining 'string'

Specifications SHOULD NOT define a string as a 'byte string'. more

The 'character string' definition SHOULD be used by most specifications. more

4.12.2 See also

Indexing strings and Choosing a definition of 'character'.

4.13 Indexing strings

The character string is RECOMMENDED as a basis for string indexing. more

A code unit string MAY be used as a basis for string indexing if this results in a significant improvement in the efficiency of internal operations when compared to the use of character string. more

Grapheme clusters MAY be used as a basis for string indexing in applications where user interaction is the primary concern. more

Specifications that define indexing in terms of grapheme clusters MUST either: (a) define grapheme clusters in terms of default grapheme clusters as defined in Unicode Standard Annex #29, Text Boundaries [UTR #29], or (b) define specifically how tailoring is applied to the indexing operation. more

Need to check the above recommendation, since extended grapheme clusters are now recommended.

The use of byte strings for indexing is NOT RECOMMENDED. more

Specifications that need a way to identify substrings or point within a string SHOULD provide ways other than string indexing to perform this operation. more

Specifications SHOULD understand and process single characters as substrings, and treat indices as boundary positions between counting units, regardless of the choice of counting units. more

Specifications of APIs SHOULD NOT specify single characters or single 'units of encoding' as argument or return types. more

When the positions between the units are counted for string indexing, starting with an index of 0 for the position at the start of the string is the RECOMMENDED solution, with the last index then being equal to the number of counting units in the string. more

4.13.2 See also

Defining 'string'.

4.14 Referring to Unicode characters

Use U+XXXX syntax to represent Unicode code points in the specification. more

The U+XXXX format is well understood when referring to Unicode code points in a specification. These are space separated when appearing in a sequence. No additional decoration is needed. Note that a code point may contain four, five, or six hexadecimal digits. When fewer than four digits are needed, the code point number is zero filled. E.g. U+0020.

4.15 Referencing the Unicode Standard

Since specifications in general need both a definition for their characters and the semantics associated with these characters, specifications SHOULD include a reference to the Unicode Standard, whether or not they include a reference to ISO/IEC 10646. more

A generic reference to the Unicode Standard MUST be made if it is desired that characters allocated after a specification is published are usable with that specification. A specific reference to the Unicode Standard MAY be included to ensure that functionality depending on a particular version is available and will not change over time. more

All generic references to the Unicode Standard MUST refer to the latest version of the Unicode Standard available at the date of publication of the containing specification. more

All generic references to ISO/IEC 10646 MUST refer to the latest version of ISO/IEC 10646 available at the date of publication of the containing specification. more

5. Resource identifiers

5.1 Basics

Resource identifiers must permit the use of characters outside those of plain ASCII. discussion

Specifications MUST define when the conversion from IRI references to URI references (or subsets thereof) takes place, in accordance with Internationalized Resource Identifiers (IRIs). more

Many current specifications already contain provisions in accordance with Internationalized Resource Identifiers (IRIs). For XML 1.0, see Section 4.2.2, External Entities. XML Schema Part 2: Datatypes provides the anyURI datatype (see Section 3.2.17). The XML Linking Language (XLink) provides the href attribute (see Section 5.4, Locator Attribute).

Document formats should allow IRIs to be used; handlers for protocols that do not currently support IRIs can convert the IRI to a URI when the IRI is dereferenced.

6. Markup & syntax

6.1 Defining elements and attributes

Do not define attribute values that will contain user readable content. Use elements for such content. more

If you do define attribute values containing user readable content, provide a means to indicate directional and language information for that text separately from the text contained in the element.

Provide a way for authors to annotate arbitrary inline content using a span-like element or construct. more

6.2 Defining identifiers

Identifiers should be case-sensitive.

6.3 Working with plain text

Avoid natural language text in elements that only allow for plain text and in attribute values.

Provide a span-like element that can be used for any text content to apply information needed for internationalization. more

Internationalization information may include a change of language, bidirectional text behaviour changes, translate flags, etc.

7. Typographic support

7.1 Enabling vertical text display

It should be possible to render text vertically for languages such as Japanese, Chinese, Korean, Mongolian, etc.

Vertical text must support line progression from LTR (eg. Mongolian) and RTL (eg. Japanese)

7.2 Setting box positioning coordinates when text direction varies

Box positioning coordinates must take into account whether the text is horizontal or vertical. more

It is typical, when localizing a user interface or web page, to create mirror-images for the RTL and LTR versions. For example, it is likely that a box that appears near the left side of a window containing English content would appear near the right side of the window if the content is Arabic or Hebrew. It should preferably automatic for this to change, based on the base direction of the current context, unless there is a strong reason for using absolute geometry. One way to achieve this is to use keywords such as start and end, rather than left and right, to indicate position.

7.3 Miscellaneous

Line heights must allow for characters that are taller than English.

Box sizes must allow for text expansion in translation.

Ruby text alongside base text should be supported for CJK text.

Line wrapping should take into account the special rules needed for non-Latin scripts. more

Various non-Latin writing systems don't simply wrap text on inter-word spaces. They have additional rules that must be respected. For example

See the CSS Text Level 3 specification for additional background. (This tutorial provides additional examples, if needed.)

Avoid specifying presentational tags, such as b for bold, and i for italic. more

It is best to avoid presentational markup b, i or u, since it isn't interoperable across writing systems and furthermore may cause unnecessary problems for localisation. In addition, some scripts have native approaches to things such as emphasis, that do not involve, and can be very different from, bolding, italicisation, etc.

In the HTML case, there was a legacy issue, but unless there is one for your specification, the recommendation is that styling be used instead to determine the presentation of the text, and that any markup or tagging should allow for general semantic approaches.

For an explanation of the issues surrounding b and i tags, see Using <b> and <i> elements.

8. Local dates, times and formats

8.1 Working with time

When defining calendar and date systems, be sure to allow for dates prior to the common era, or at least define handling of dates outside the most common range.

When defining time or date data types, ensure that the time zone or relationship to UTC is always defined.

Provide a health warning for conversion of time or date data types that are "floating" to/from incremental types, referring as necessary to the Time Zones WG Note. more

Allow for leap seconds in date and time data types. more

These occur occasionally when the number of seconds in a minute is allowed to range from 0 to 60 (ie. there are sixty-ONE seconds in that minute).

Use consistent terminology when discussing date and time values. Use 'floating' time for time zone independent values.

Keep separate the definition of time zone from time zone offset.

Use IANA time zone IDs to identify time zones. Do not use offsets or LTO as a proxy for time zone.

Use a separate field to identify time zone.

When defining rules for a "week", allow for culturally specific rules to be applied. more

For example, the weekend is not always Saturday/Sunday; the first day of week is not always Sunday [or Monday or...], etc.

When defining rules for week number of year, allow for culturally specific rules to be applied.

When non-Gregorian calendars are permitted, note that the "month" field can go to 13 (undecimber).

8.2 Designing forms

When defining email field validation, allow for EAI (smtputf8) names.

8.3 Working with numbers

When parsing user input of numeric values, allow for digit shaping (non-ASCII digits).

When formatting numeric values for display, allow for culturally sensitive display, including the use of non-ASCII digits (digit shaping).

A. References


B. Revision Log

The following summarises substantive changes since the previous publication, but the material is still subject to significant flux as it develops. This should not be a reason not to use the document. What it so far contains is useful, and any shortfalls can be reported or discussed.

  1. A variety of additional guidelines were added after working through previous review comments.
  2. Structure of the document changed to better reflect the world as seen by spec developers.
  3. Substantial improvements to the 'Resource' and 'Block' sections, based on a recent review.

See the github commit log for more details.

C. Acknowledgements

Thanks to Addison Phillips for help reviewing old reviews for recommendations.

Other people who contributed through reviews or issues include Steve Atkin, Andrew Cunnigham, Martin Dürst, Asmus Freytag, John Klensin, Tomer Mahlin, Florian Rivoal.