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.
Sending comments on this document
If you wish to make comments regarding this document, please raise them as github issues. Only send comments by email if you are unable to raise issues on github (see links below).
To make it easier to track comments, please raise separate issues or emails for each comment, and point to the section you are commenting on using a URL for the dated version of the document. All comments are welcome.
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.
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.
The guidelines in this document are not intended to be hard and fast requirements. This document will achieve a significant part of its purpose if, where you don't understand the guidelines or disagree with them, you contact the Internationalization WG to discuss what should be done.
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.
If your spec is github based, you can now create a snapshot of the checklist items in markdown. If you add that to a github issue, you can check off items that are ok, and add comments while doing a self-review. Generate the code here.
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.
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
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.
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 to identify the text processing language, rather than creating a new attribute or mechanism. more
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.
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.
It should be possible to associate a metadata-type language declaration (which indicates the intended use of the resource rather than the language of a specific range of text) with multiple language values. more
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.
Attributes that express the language of external resources should not use the HTML lang and XML xml:lang language attributes, but should use a different attribute when they represent metadata (which indicates the intended use of the resource rather than the language of a specific range of text). more
Using a different attribute to indicate the language of an external resource allows the attribute to specify more than one language. It also works better if the resource pointed to is not in a single language.
This distinction can be seen in HTML in the separation of the lang and hreflang attributes. The former indicates the language of the text within the HTML page; the latter is metadata indicating the expected language of a page that is linked to.
For a longer discussion of this see xml:lang in XML document schemas. This article talks specifically about xml:lang, but the concepts are applicable to other situations.
xml:lang in XML document schemas
When should I use xml:lang and when should I define my own element or attribute for passing language values in an XML document schema (DTD)?
Why use the language attribute?
Describes why it is useful to use the lang or xml:lang attribute to label language in web pages.
Use cases for language information in web annotations
Description of use cases for annotations that illustrate the differences between text-processing and metadata types of language declaration.
HTTP headers, meta elements and language information
How the distinction between text-processing language and language metadata plays out in HTML5.
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.)
An overview of how to create language tags using BCP 47.
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
Use cases for language information in web annotations
Description of use cases for annotations that illustrate the differences between text-processing and metadata types of language declaration.
HTTP headers, meta elements and language information
How the distinction between text-processing language and language metadata plays out in HTML5.
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 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
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.
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.
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.
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.
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.
There are a number of possible ways of setting the base direction.
ltr, rtl or auto.
dir=auto on an HTML element.)dir attributes in your HTML file.)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.auto, since HTML specifies a default direction.)
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.
Embedded ranges of text within a single paragraph may need to have a different base direction. For example,
"The title was '!NOITASILANOITANRETNI'."
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.
Reasons to avoid relying on control characters to set direction include the following:
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.
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.)
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:
auto value with language tags.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.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
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.
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.
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.
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.
The term character is often used to mean different things in different contexts: it can variously refer to the visual, logical, or byte-level representation of a given piece of text. This makes the term too imprecise to use when specifying algorithms, protocols, or document formats. Understanding how characters are defined and encoded in computing systems, along with the associated terminology used to make such specification unambiguous, is thus a necessary prerequisite to discussing the processing of string data.
The visual manifestation of a "character"—the shape most people mean when they say "character"—is what we call a user-perceived character. These visual building blocks are usually perceived to be a single unit of the visible text.
At their simplest, user-perceived characters are a single shape that can be tied one-to-one to the underlying computing representation. But a user-perceived character can be formed, in some scripts, from more than one character. And a given logical character can take many different shapes due to such influences as font selection, style, or the surrounding context (such as adjacent characters). In some cases, a single user-perceived character might be formed from a long sequence of logical characters. And some logical characters (so-called "combining marks") are always used in conjunction with another character.
When user-perceived characters are represented visibly (on screen or in print), they are represented by individual rendering units. This visual unit is called a grapheme (the word glyph is also used). Graphemes are the visual units found in fonts and rendering software.
Graphemes are encoded into computer systems using "logical characters". A character set is a set of logical characters: a specific collection of characters that can be used together to encode text. The most important character set is the Universal Character Set, also known as [[!Unicode]]. This character set includes all of the characters used to encode text, including historical or extinct writing systems as well as modern usage, private use, typesetting symbols, and other things, such as the emoji. Other character sets are defined subsets of Unicode. In Unicode, a 'character' is a single abstract logical unit of text. Each character in Unicode is assigned a unique integer number between 0x0000 and 0x10FFFF, which is called its code point. The term code point therefore unambiguously refers to a single logical character and its integer representation.
Specifications SHOULD explicitly define the term 'character' to mean a Unicode code point.
The relationship between code points and graphemes can be complex. In most cases, a code point sequence that forms a single grapheme should be treated as a single textual unit. For example, when cursoring across text, an entire grapheme should select together. It shouldn't be possible to cursor into the "middle" of a grapheme or delete only a part of user-perceived character. Because the relationship is not one-to-one between code points and graphemes and because the relationship can be somewhat complex, [[!Unicode]] defines a specific type of grapheme: the extended grapheme cluster which most closely matches the mapping of the underlying logical character sequence to a user-perceived character. When referring to 'graphemes' in this document, we mean extended grapheme clusters (unless otherwise called out).
Another example of the complex relationship between code points and graphemes are certain emoji. The emoji character for "family" has a code point in Unicode: 👪 [U+1F46A FAMILY]. It can also be formed by using using a sequence of code points: U+1F468 U+200D U+1F469 U+200D U+1F466. Altering or adding other emoji characters can alter the composition of the family. For example the sequence 👨👩👧👧 U+1F468 U+200D U+1F469 U+200D U+1F467 U+200D U+1F467 results in a composed emoji character for a "family: man, woman, girl, girl" on systems that support this kind of composition. Many common emoji can only be formed using sequences of code points, but should be treated as a single user-perceived character when displaying or processing the text. You wouldn't want to put a line-break in the middle of the family!
Unicode code points are just abstract integer values: they are not the values actually present in the memory of the computer or serialized on the wire. When processing text, computers use an array of fixed-size integer units. One such common unit is the byte (or octet, since bytes have 8 bits per unit). There are also 16-bit, 32-bit, or other size units. In many programming languages, the unit is called a char, which suggests that strings are made of "characters". We use the term code unit to refer unambiguously to the programming and serialized representation of characters. For example, in C, a char is generally an 8-bit byte: each char is a 8-bit code unit. In Java or Javascript, a char is a 16-bit value.
A set of rules for converting code points to or from code units is called a character encoding form (or just "character encoding" for short.
Specifications SHOULD use specific terms, when available, instead of the general term 'character'. more
When specifications use the term 'character' the specifications MUST define which meaning they intend, and SHOULD explicitly define the term 'character' to mean a Unicode code point. more
Specifications, software and content MUST NOT require or depend on a one-to-one relationship between characters and units of physical storage. more
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
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
In W3C Recommendation, Character Model for the World Wide Web.
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
Digital Encoding of Characters
In W3C Recommendation, Character Model for the World Wide Web.
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
Digital Encoding of Characters
In W3C Recommendation, Character Model for the World Wide Web.
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
In W3C Recommendation, Character Model for the World Wide Web.
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
Choice and identification of code points
In W3C Recommendation, Character Model for the World Wide Web.
What is the 'Document Character Set' for XML and HTML, and how does it relate to the encodings I use for my documents?
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
Choice and identification of code points
In W3C Recommendation, Character Model for the World Wide Web.
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
In W3C Recommendation, Character Model for the World Wide Web.
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
Visual rendering and logical order
In W3C Recommendation, Character Model for the World Wide Web.
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
In W3C Recommendation, Character Model for the World Wide Web.
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
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
In W3C Working Draft, Character Model for the World Wide Web: String Matching and Searching.
Specifications SHOULD NOT define a string as a 'byte string'. more
The 'character string' definition SHOULD be used by most specifications. more
In W3C Recommendation, Character Model for the World Wide Web.
Some specifications, formats, or protocols or their implementations need to specify limits for the size of a given data structure or text field. This could be due to many reasons, such as limits on processing, memory, data structure size, and so forth. When selecting or specifying limits on the length of a given string, specifications or implementations need to ensure that they do not cause corruption in the text.
Specifications SHOULD NOT limit the size of data fields unless there is a specific practical or technical limitation.
There are many reasons why a length limit might be needed in a specification or format. Generally length limits correspond to underlying limits in the implementation, such as the use of fixed-size fields in a database or data store, the desire to fit into practical boundaries such as packet size, or some other implementation detail related to storage allocation or efficiency.
When truncating strings, it's necessary to decide what units to use when counting the size of the string. In many cases this is beyond the control of the specification, since the truncation is occuring for some preordained reason. However, when the choice is available, some general guidelines can be applied.
If the limitation is related to the number of display positions, the grapheme count usually corresponds most closely to the expected limit. Note that proportional width fonts, combining marks, complex scripts, and many other factors complicate counting "screen positions". In Web pages, for example, the CSS text-overflow property provides visual truncation without disturbing the content of the text. Attempts to estimate the size of a given piece of text based on the number of Unicode code points or even the number of grapheme clusters is mostly futile.
Otherwise most limits are expressed in terms of code points in Unicode or code units (such as bytes) in a specific character encoding. Code points provides the best user experience, since all Unicode code points are treated identically: if text is truncated after 40 code points, all languages and scripts get the same number of code points to work with. By contrast, when the size limit is expressed in code units such as bytes in UTF-8, users who write in a language that mostly uses ASCII letters get many more characters (code points) for a given size limit than user's whose language is mostly made up of characters that take 2-, 3-, or 4-bytes per code point.
Specifications that limit the length of a string MUST specify which type of unit (extended grapheme clusters, Unicode code points, or code units) the length limit uses.
Specifications that limit the length of a string SHOULD specify the length in terms of Unicode code points.
If a specification sets a length limit in code units (such as bytes), it MUST specify that truncation can only occur on code point boundaries.
If a specification specifies a length limit, it SHOULD specify that any string that is truncated include an indicator, such as ellipses, that the string has been altered.
When specifying a length limitation in code units (such as bytes), specifications SHOULD set the maximum length in a way that accommodates users whose language requires multibyte code unit sequences.
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
In W3C Recommendation, Character Model for the World Wide Web.
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.
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
Referencing the Unicode Standard and ISO/IEC 10646
In W3C Recommendation, Character Model for the World Wide Web.
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.
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
Identifiers should be case-sensitive.
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.
Text decoration such as underline and overline should allow lines to skip ink.
It should be possible to specify the distance of overlines and underlines from the text. more
Skipping ink for text decoration such as underlines may not be appropriate for some scripts, such as Arabic, which prefers to move the underline further away from the baseline instead.
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)
By default, text decoration, ruby, and the like in vertical text where lines are stacked from left to right (eg. Mongolian) should appear on the same side as for CJK vertical text. Placement should not rely on the before and after line locations.
Vertical writing modes that are equivalent to the vertical- values in CSS (only) should use UTR50 to apply default text orientation of characters. (This does not apply to writing modes that are equivalent to sideways- in CSS.)
By default, glyphs of scripts that are normally horizontal should run along a line in vertical text such that the top of the character is toward the right side of the vertical line, but there should also be a mechanism to allow them to progress down the line in upright orientation. Such a mechanism should use grapheme clusters as a minimum text unit, but where necessary allow syllabic clusters to be treated as a unit when they involve more than one grapheme cluster.
Upright Arabic text in vertical lines should use isolated letter forms and the order of text should read top to bottom.
It should be possible for some sequences of characters (particularly digits) to run horizontally within vertical lines (tate chu yoko).
Writing modes should provide values like sideways-lr and sideways-rl in CSS to allow for vertical rotation of lines of horizontal script text. UTR50 is not applicable for these cases.
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.
'Ruby' style annotations alongside base text should be supported for Chinese, Japanese, Korean and Mongolian text, in both horizontal and vertical writing modes.
Ruby implementations should support zhuyin fuhao (bopomofo) ruby for Traditional Chinese.
Ruby implementations should support a tabular content model (such that ruby contents can be arranged in a sequence approximating to rb rb rt rt).
Ruby implementations should make it possible to use an explicit rb tag for ruby bases.
Ruby implementations should allow annotations to appear on either or both sides of the base text.
What are 'ruby' annotations?
Line heights must allow for characters that are taller than English.
Box sizes must allow for text expansion in translation.
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.
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).
Check whether you really need to store or access given name and family name separately. more
Avoid placing limits on the length of names, or if you do, make allowance for long strings. more
Try to avoid using the labels 'first name' and 'last name' in non-localized contexts. more
Consider whether it would make sense to have one or more extra fields, in addition to the full name field, where users can provide part(s) of their name that you need to use for a specific purpose. more
Allow for users to be asked separately how they would like you be addressed when someone contacts them. more
If parts of a person's name are captured separately, ensure that the separate items can capture all relevant information. more
Be careful about assumptions built into algorithms that pull out the parts of a name automatically. more
Don't assume that a single letter name is an initial. more
Don't require that people supply a family name. more
Don't forget to allow people to use punctuation such as hyphens, apostrophes, etc. in names. more
Don't require names to be entered all in upper case. more
Allow the user to enter a name with spaces. more
Don't assume that members of the same family will share the same family name. more
It may be better for a form to ask for 'Previous name' rather than 'Maiden name' or 'née'. more
You may want to store the name in both Latin and native scripts, in which case you probably need to ask the user to submit their name in both native script and Latin-only form, as separate items. more
Personal names around the world
How do people's names differ around the world, and what are the implications of those differences on the design of forms, databases, ontologies, etc. for the Web?
When defining email field validation, allow for EAI (smtputf8) names.
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).
None.
You can take a snapshot of the checklist, and paste it into an issue in your Github repository. Then you can click on a checkbox alongside a checklist item to indicate whether or not your spec supports it, or you can add your own notes/questions by editing the issue.
To do this, simply click on the button just below, then paste the output it generates into a new issue in your repository.
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.
See the github commit log for more details.
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, Chaals McCathieNevile, Florian Rivoal.