The goal of this document is to facilitate use of the Web by all
people, regardless of their language, script, writing system, and cultural
conventions, in accordance with the
W3C goal of universal
access. One basic prerequisite to achieve this goal is to be able to
transmit and process the characters used around the world in a well-defined and
well-understood way.
The main target audience of this document is W3C specification
developers. This document defines conformance requirements for other W3C
specifications. This document and parts of it can also be referenced from other
W3C specifications.
Other audiences of this document include software developers,
content developers, and authors of specifications outside the W3C. Software
developers and content developers implement and use W3C specifications. This
document defines some conformance requirements for software developers and
content developers that implement and use W3C specifications. It also helps
software developers and content developers to understand the character-related
provisions in other W3C specifications.
The character model described in this document provides authors of
specifications, software developers, and content developers with a common
reference for consistent, interoperable text manipulation on the World Wide
Web. Working together, these three groups can build a more international
Web.
Topics addressed include encoding identification, early uniform
normalization, string identity matching, string indexing, and URI conventions.
Some introductory material on characters and character encodings is also
provided.
Topics not addressed or barely touched include collation (sorting),
fuzzy matching and language tagging. Some of these topics may be addressed in a
future version of this specification.
At the core of the model is the Universal Character Set (UCS),
defined jointly by The Unicode Standard [Unicode] and ISO/IEC
10646 [ISO/IEC 10646]. In this document, Unicode is used
as a synonym for the Universal Character Set. The model will allow Web
documents authored in the world's scripts (and on different platforms) to be
exchanged, read, and searched by Web users around the world.
All W3C specifications must conform to this document (see section
2 Conformance). Authors of other specifications (for
example, IETF specifications) are strongly encouraged to take guidance from
it.
Since other W3C specifications will be based on some of the
provisions of this document, without repeating them, software developers
implementing W3C specifications must conform to these provisions.
This section provides some historical background on the topics
addressed in this document.
Starting with Internationalization of the Hypertext Markup
Language [RFC 2070], the Web community has recognized
the need for a character model for the World Wide Web. The first step towards
building this model was the adoption of Unicode as the document character set
for HTML.
The choice of Unicode was motivated by the fact that Unicode:
-
is the only universal character repertoire available,
-
covers the widest possible range,
-
provides a way of referencing characters independent of the
encoding of a resource,
-
is being updated/completed carefully,
-
is widely accepted and implemented by industry.
W3C adopted Unicode as the document character set for HTML in
[HTML 4.0]. The same approach was later used for specifications
such as XML 1.0 [XML 1.0] and CSS2 [CSS2]. Unicode
now serves as a common reference for W3C specifications and applications.
The IETF has adopted some policies on the use of character sets on
the Internet (see [RFC 2277]).
When data transfer on the Web remained mostly unidirectional (from
server to browser), and where the main purpose was to render documents, the use
of Unicode without specifying additional details was sufficient. However, the
Web has grown:
-
Data transfers among servers, proxies, and clients, in all
directions, have increased.
-
Non-ASCII characters [MIME] are being used in
more and more places.
-
Data transfers between different protocol/format elements
(such as element/attribute names, URI components, and textual content) have
increased.
-
More and more APIs are defined, not just protocols and
formats.
In short, the Web may be seen as a single, very large application
(see [Nicol]), rather than as a collection of small independent
applications.
While these developments strengthen the requirement that Unicode be
the basis of a character model for the Web, they also create the need for
additional specifications on the application of Unicode to the Web. Some
aspects of Unicode that require additional specification for the Web include:
-
Choice of encoding forms (UTF-8, UTF-16, UTF-32).
-
Counting characters, measuring string length in the presence
of variable-length encodings and combining characters).
-
Duplicate encodings (e.g. precomposed vs decomposed).
-
Use of control codes for various purposes (e.g.
bidirectionality control, symmetric swapping, etc.).
It should be noted that such properties also exist in legacy
encodings (where legacy encoding is taken to mean any character
encoding not based on Unicode), and in many cases have been inherited by
Unicode in one way or another from such legacy encodings.
The remainder of this document presents additional specifications
and requirements to ensure an interoperable character model for the Web, taking
into account earlier work (from W3C, ISO and IETF).
For the purpose of this specification, the producer of
text data is the sender of the data in the case of protocols, and the tool that
produces the data in the case of formats. The recipient of text
data is the software module that receives the data.
NOTE: A software module may be both a recipient and a producer.
Unicode code points are denoted as U+hhhh, where "hhhh" is a
sequence of at least four, and at most six hexadecimal digits.
In this document, requirements are expressed using the key words
"MUST", "MUST NOT",
"REQUIRED", "SHALL" and "SHALL
NOT". Recommendations are expressed using the key words
"SHOULD", "SHOULD NOT" and
"RECOMMENDED". "MAY" and
"OPTIONAL" are used to indicate optional features or
behaviour. These keywords are used in accordance with RFC 2119
[RFC 2119].
This specification places conformance requirements on specifications,
on software and on Web content. To aid the reader, all requirements are
preceded by '[X]' where 'X' is one of 'S' for specifications, 'I' for software
implementations, and 'C' for Web content. These markers indicate the relevance
of the requirement and allow the reader to quickly locate relevant requirements
using the browser's search function.
[S] [I] [C] In
order to conform to this document, specifications MUST NOT
violate any requirements preceded by [S], software MUST NOT
violate any requirements preceded by [I], and content MUST
NOT violate any requirements preceded by [C].
[S] Every W3C specification
MUST:
-
conform to the requirements applicable to specifications,
-
specify that implementations MUST conform to
the requirements applicable to software, and
-
specify that content created according to that specification
MUST conform to the requirements applicable to content.
[S] If an existing W3C specification
does not conform to the requirements in this document, then the next version of
that specification MUST be modified in order to
conform.
[I] Where this specification contains
a procedural description, it MUST be understood as a way to
specify the desired external behavior. Implementations MAY
use other ways of achieving the same results, as long as observable behavior is
not affected.
The glossary entry in [Unicode 3.0] gives:
"Character. (1) The smallest component of written language
that has semantic values; refers to the abstract meaning and/or shape
..."
The word 'character' is used in many contexts, with
different meanings. Human cultures have radically differing writing systems,
leading to radically differing concepts of a character. Such wide variation in
end user experience can, and often does, result in misunderstanding. This
variation is sometimes mistakenly seen as the consequence of imperfect
technology. Instead, it derives from the great flexibility and creativity of
the human mind and the long tradition of writing as an important part of the
human cultural heritage. The alphabetic approach used by scripts such as Latin,
Cyrillic and Greek is only one of several possibilities.
EXAMPLE: Japanese
hiragana and katakana are syllabaries. A character in these scripts corresponds
to a syllable (usually a combination of consonant plus vowel).
EXAMPLE: Korean Hangul is a featural syllabary that combines symbols for
individual sounds of the language into square syllabic blocks. Depending on the
user and the application, either the individual symbols or the syllabic
clusters can be considered to be characters.
EXAMPLE: Indic scripts
are abugidas. Each consonant letter carries an inherent vowel that is
eliminated or replaced using semi-regular or irregular ways to combine
consonants and vowels into clusters. Depending on the user and the application,
either individual consonants or vowels, or the consonant or consonant-vowel
clusters can be perceived as characters.
EXAMPLE: Arabic script is
an example of an abjad. Short vowel sounds are typically not written at all.
When they are written they are indicated by the use of combining marks placed
above and below the consonantal letters.
The developers of W3C specifications, and the developers of
software based on those specifications, are likely to be more familiar with
usages they have experienced and less familiar with the wide variety of usages
in an international context. Furthermore, within a computing context,
characters are often confused with related concepts, resulting in incomplete or
inappropriate specifications and software.
This section examines some of these contexts, meanings and
confusions.
In some scripts, characters have a close relationship to phonemes
(a phoneme is a minimally distinct sound in the context of a
particular spoken language), while in others they are closely related to
meanings. Even when characters (loosely) correspond to phonemes, this
relationship may not be simple, and there is rarely a one-to-one correspondence
between character and phoneme.
EXAMPLE: In the English sentence,
"They were too close to the door to close it." the same character
's' is used to represent both /s/ and /z/ phonemes.
EXAMPLE: In many scripts a single character may represent a sequence of
phonemes, such as the syllabic characters of Japanese hiragana.
EXAMPLE: In many writing systems a sequence of characters may represent a
single phoneme, for example 'wr' and 'ng' in
"writing".
[S] [I] Specifications
and software MUST NOT assume that there is a one-to-one
correspondence between characters and the sounds of a
language.
Visual rendering introduces the notion of a glyph.
Glyphs are defined by ISO/IEC 9541-1 [ISO/IEC 9541-1] as
"a recognizable abstract graphic symbol which is independent of a
specific design". There is not a one-to-one correspondence
between characters and glyphs:
-
A single character can be represented by multiple glyphs
(each glyph is then part of the representation of that character). These glyphs
may be physically separated from one another.
-
A single glyph may represent a sequence of characters (this
is the case with ligatures, among others).
-
A character may be rendered with very different glyphs
depending on the context.
-
A single glyph may represent different characters (e.g.
capital Latin A, capital Greek A and capital Cyrillic A).
Each glyph can be represented by a number of different glyph
images; a set of glyph images makes up a font. Glyphs can be
construed as the basic units of organization of the visual rendering of text,
just as characters are the basic unit of organization of encoded text.
[S] [I] Specifications
and software MUST NOT assume a one-to-one mapping between
character codes and units of displayed text.
See A Examples of Characters, Keystrokes and
Glyphs for examples of the
complexities of character to glyph mapping.
Some scripts, in particular Arabic and Hebrew, are written from
right to left. Text including characters from these scripts can run in both
directions and is therefore called bidirectional text (see example
A.6 in Appendix A). The Unicode
Standard [Unicode] requires that characters be stored and
interchanged in logical order. [S] Protocols,
data formats and APIs MUST store, interchange or process
text data in logical order.
In the presence of bidi text, two possible
selection modes must be considered. The first is logical selection
mode, which selects all the characters logically located
between the end-points of the user's mouse gesture. Here the user selects from
between the first and second letters of the second word to the middle of the
number. Logical selection looks like this:
It is a consequence of the bidirectionality of the text that a
single, continuous logical selection in memory results in a discontinuous
selection appearing on the screen. This discontinuity, as well as the
somewhat unintuitive behavior of the cursor, makes some users prefer a
visual selection mode, which selects all the characters
visually located between the end-points of the user's mouse
gesture. With the same mouse gesture as before, we now obtain:
In this mode, a single visual selection range results in
two logical ranges, which have to be accommodated by protocols,
APIs and implementations.
[S] Specifications of protocols
and APIs that involve selection of ranges SHOULD provide for
discontiguous selections, at least to the extent necessary to support
implementation of visual selection on screen on top of those protocols and
APIs.
In keyboard input, it is not always the case that
keystrokes and input characters correspond one-to-one. A limited number of keys
can fit on a keyboard. Some keyboards will generate multiple characters from a
single keypress. In other cases ('dead keys') a key will generate
no characters, but affect the results of subsequent keypresses. Many writing
systems have far too many characters to fit on a keyboard and must rely on more
complex input methods, which transform keystroke sequences into
character sequences. Other languages may make it necessary to input some
characters with special modifier keys. See A Examples of Characters, Keystrokes and
Glyphs
for examples of non-trivial input.
[S] [I] Specifications
and software MUST NOT assume that a single keystroke results
in a single character, nor that a single character can be input with a single
keystroke (even with modifiers), nor that keyboards are the same all over the
world.
String comparison as used in sorting and searching is based on
units which do not in general have a one-to-one relationship to encoded
characters. Such string comparison can aggregate a character sequence into a
single collation unit with its own position in the sorting order,
can separate a single character into multiple collation units, and can
distinguish various aspects of a character (case, presence of diacritics, etc.)
to be sorted separately (multi-level sorting).
In addition, a certain amount of pre-processing may also be
required, and in some scripts (such as Japanese and Arabic) sort order is
governed by higher order factors such as phonetics or word roots. Collation
methods may also vary by application (e.g. dictionaries may be sorted
differently than telephone books).
EXAMPLE: In traditional Spanish sorting, the letter sequences
'ch' and 'll' are treated as atomic collation units.
Although Spanish sorting, and to some extent Spanish everyday use, treat
'ch' as a single unit, current digital encodings treat it as two
letters, and keyboards do the same (the user types 'c', then
'h').
EXAMPLE: In most languages, the letter
'æ' is sorted as two consecutive collation units: 'a'
and 'e'.
EXAMPLE: The sorting of text written in a
bicameral script (i.e. a script which has distinct upper and lower case
letters) is usually required to ignore case differences in a first pass; case
is then used to break ties in a later pass.
EXAMPLE: Treatment of
accented letters in sorting is dependent on the script or language in question.
The letter 'ö' is treated as a modified 'o' in
French, but as a letter completely independent from 'o' (and
sorting after 'z') in Swedish. In German certain applications
treat the letter 'o' as if it were the sequence
'oe'.
EXAMPLE: In Thai the sequence U+0E44 U+0E01 must
be sorted as if it was written U+0E01 U+0E44. Reordering is typically done
during an initial pre-processing stage.
[S] [I] Software
that sorts or searches text for users MUST do so on the
basis of appropriate collation units and ordering rules for the relevant
language and/or application.
Computer storage and communication rely on units of physical
storage and information interchange, such as bits and bytes (also known as
octets, as nowadays the word bytes is generally considered to mean 8-bit
bytes). A frequent error in specifications and implementations is the equating
of characters with units of physical storage. The mapping between characters
and such units of storage is actually quite complex, and is discussed in the
next section, 3.2 Digital Encoding of Characters.
[S] [I] Specifications
and software MUST NOT assume a one-to-one relationship
between characters and units of physical storage.
The term character is used differently in a variety
of contexts and often leads to confusion when used outside of these contexts.
In the context of the digital representations of text, a character can be
defined informally as a small logical unit of text. Text is then
defined as sequences of characters. While such an informal definition is
sufficient to create or capture a common understanding in many cases, it is
also sufficiently open to create misunderstandings as soon as details start to
matter. In order to write effective specifications, protocol implementations,
and software for end users, it is very important to understand that these
misunderstandings can occur.
[S] When specifications use the
term 'character' it MUST be clear which of the
possible meanings they intend.
[S] Specifications SHOULD
avoid the use of the term 'character' if a more specific term is
available.
To be of any use in computers, in computer communications and in
particular on the World Wide Web, characters must be encoded. In fact, much of
the information processed by computers over the last few decades has been
encoded text, exceptions being images, audio, video and numeric data. To
achieve text encoding, a large variety of encoding schemes have been devised,
which can loosely be defined as mappings between the character sequences that
users manipulate and the sequences of bits that computers manipulate.
Given the complexity of text encoding and the large variety of
schemes for character encoding invented throughout the computer age, a more
formal description of the encoding process is useful. The process of defining a
text encoding can be described as follows (see [UTR #17] for a more
detailed description):
-
A set of characters to be encoded is identified. The
characters are pragmatically chosen to express text and to efficiently allow
various text processes in one or more target languages. They may not correspond
precisely to what users perceive as letters and other characters. The set of
characters is called a repertoire.
-
Each character in the repertoire is then associated with a
(mathematical, abstract) non-negative integer, the code point
(also known as a character number or code position).
The result, a mapping from the repertoire to the set of non-negative integers,
is called a coded character set (CCS).
-
To enable use in computers, a suitable base datatype is
identified (such as a byte, a 16-bit unit of storage or other) and a
character encoding form (CEF) is used, which encodes the abstract
integers of a CCS into sequences
of the code units of the base datatype. The encoding form can be
extremely simple (for instance, one which encodes the integers of the
CCS into the natural
representation of integers of the chosen datatype of the computing platform) or
arbitrarily complex (a variable number of code units, where the value of each
unit is a non-trivial function of the encoded integer).
-
To enable transmission or storage using byte-oriented devices,
a serialization scheme or character encoding scheme
(CES) is next used. A CES is a mapping of the code units
of a CEF into well-defined
sequences of bytes, taking into account the necessary specification of
byte-order for multi-byte base datatypes and including in some cases switching
schemes between the code units of multiple
CESes (an example is ISO
2022). A CES, together
with the CCSes it is used with,
is identified by an IANA charset identifier.
Given a sequence of bytes representing text and a charset identifier,
one can in principle unambiguously recover the sequence of characters of the
text.
NOTE: The term 'character encoding' is somewhat ambiguous,
as it is sometimes used to describe the actual process of encoding characters
and sometimes to denote a particular way to perform that process (as in
"this file is in the X character encoding"). Context normally
allows the distinction of those uses, once one is aware of the ambiguity.
NOTE: Unfortunately, there are some important cases of charset
identifiers that denote a range of slight variants of an encoding scheme, where
the differences may be crucial (e.g. the well-known yen/backslash case) and may
vary over time. In those cases, recovery of the character sequence from a byte
sequence is not totally unambiguous. See the
[XML Japanese Profile] for examples of such ambiguous
charsets.
In very simple cases, the whole encoding process can be collapsed to
a single step, a trivial one-to-one mapping from characters to bytes; this is
the case, for instance, for US-ASCII [MIME] and ISO-8859-1.
Text data is said to be in a
Unicode encoding form if it is encoded in UTF-8, UTF-16 or
UTF-32.
Transcoding is the process of converting text data from
one
Character Encoding Form to another.
Transcoders work only at the level of character encoding and do not parse the
text; consequently, they do not deal with character escapes such as numeric
character references (see 3.7 Character Escaping) and do not adjust
embedded character encoding information (for instance in an XML declaration or
in an HTML meta element).
NOTE: Transcoding may involve one-to-one, many-to-one, one-to-many or
many-to-many mappings. In addition, the storage order of characters varies
between encodings: some, such as Unicode, prescribe logical ordering while
others use visual ordering; among encodings that have separate diacritics, some
prescribe that they be placed before the base character, some after. Because of
these differences in sequencing characters, transcoding may involve reordering:
thus XYZ may map to yxz.
A normalizing
transcoder is a transcoder that converts from a legacy encoding to a
Unicode encoding form and ensures that the result is in Unicode
Normalization Form C (see 4.2.1 Unicode-normalized Text). For most
legacy encodings, it is possible to construct a normalizing transcoder; it is
not possible to do so if the encoding's repertoire contains characters not in
Unicode.
Various specifications use the notion of a 'string',
sometimes without defining precisely what is meant and sometimes defining it
differently from other specifications. The reason for this variability is that
there are in fact multiple reasonable definitions for a string, depending on
one's intended use of the notion; the term 'string' is used for
all these different notions because these are actually just different views of
the same reality: a piece of text stored inside a computer. This section
provides specific definitions for different notions of 'string'
which may be reused elsewhere.
Byte string: A string viewed as a
sequence of bytes representing characters in a particular encoding. This
corresponds to a CES. As a definition for a
string, this definition is most often useless, except when the textual nature
is unimportant and the string is considered only as a piece of opaque data with
a length in bytes. [S] Specifications in
general SHOULD NOT define a string as a 'byte
string'.
Code unit string: A string
viewed as a sequence of code units representing characters in a particular
encoding. This corresponds to a CEF. This
definition is useful in APIs that expose a physical representation of string
data. Example: For the DOM [DOM Level 1], UTF-16 was chosen based on
widespread implementation practice.
Character string: A string
viewed as a sequence of characters, each represented by a code point in Unicode
[Unicode]. This is usually what programmers consider to be a
string, although it may not match exactly what most users perceive as
characters. This is the highest layer of abstraction that ensures
interoperability with very low implementation effort.
[S] The 'character string'
definition of a string is generally the most useful and
SHOULD be used by most specifications, following the
examples of Production [2] of XML 1.0 [XML 1.0], the SGML
declaration of HTML 4.0 [HTML 4.01], and the character model of RFC
2070 [RFC 2070].
EXAMPLE: Consider the
string 
comprising the characters U+10333 GOTHIC LETTER DAGS, U+2260
NOT EQUAL TO and U+0030 DIGIT ZERO, encoded in
UTF-16 in big-endian byte order. The rows of the following table show the
string viewed as a character string, code unit string and byte string,
respectively:
Many Internet protocols and data formats, most notably the very
important Web formats HTML, CSS and XML, are based on text. In those formats,
everything is text but the relevant specifications impose a structure on the
text, giving meaning to certain constructs so as to obtain functionality in
addition to that provided by plain text. HTML and XML are markup
languages, defining entities entirely composed of text but with
conventions allowing the separation of this text into markup and
character data. Citing from the XML 1.0 specification
[XML 1.0],
section
2.4:
"Text consists of intermingled character data and markup.
[...] All text that is not markup constitutes the character data
of the document."
For the purposes of this section, the important aspect is that
everything is text, that is, a sequence of characters.
Since its early days, the Web has seen the development of a
Reference Processing Model, first described for HTML in RFC 2070
[RFC 2070]. This model was later embraced by XML and CSS. It is
applicable to any data format or protocol that is text-based as described
above. The essence of the Reference Processing Model is the use of Unicode as a
common reference. Use of the Reference Processing Model by a specification does
not, however, require that implementations actually use Unicode. The
requirement is only that the implementations behave as if the processing took
place as described by the Model.
A specification conforms to the Reference Processing Model if all of
the following apply:
-
[S ] Specifications
MUST be defined in terms of Unicode characters, not bytes or
glyphs.
-
[S] Specifications
SHOULD allow the use of the full range of Unicode code
points from 0 to 0x10FFFF inclusive; any exceptions SHOULD
be listed and justified; code points above 0x10FFFF SHOULD
NOT be used.
-
[S] Specifications
MAY allow use of any character encoding which can be
transcoded to Unicode for its text entities.
-
[S] Specifications
MAY choose to disallow or deprecate some encodings and to
make others mandatory. Independent of the actual encoding, the specified
behavior MUST be the same as if the processing
happened as follows:
-
The encoding of any text entity received by the
application implementing the specification MUST be
determined and the text entity MUST be interpreted as a
sequence of Unicode characters - this MUST be equivalent to
transcoding the entity to some Unicode encoding form, adjusting any character
encoding label if necessary, and receiving it in that Unicode encoding
form.
-
All processing MUST take place on this
sequence of Unicode characters.
-
If text is output by the application, the sequence of
Unicode characters MUST be encoded using an encoding chosen
among those allowed by the specification.
-
[S] If a specification is such
that multiple text entities are involved (such as an XML document referring to
external parsed entities), it MAY choose to allow these
entities to be in different character encodings. In all cases, the
Reference Processing Model
MUST be applied to all entities.
[S] All specifications that involve
text MUST specify processing according to the
Reference Processing
Model.
NOTE: All specifications that derive from the XML 1.0 specification
[XML 1.0] automatically inherit this Reference Processing Model.
XML is entirely defined in terms of Unicode characters and mandates the UTF-8
and UTF-16 encodings while allowing any other encoding for parsed entities.
NOTE: When specifications choose to allow encodings other than Unicode
encodings, implementers should be aware that the correspondence between the
characters of a legacy encoding and Unicode characters may in practice depend
on the software used for transcoding. See the Japanese XML Profile
[XML Japanese Profile] for examples of such inconsistencies.
Because encoded text cannot be interpreted and
processed without knowing the encoding, it is vitally important that the
character encoding scheme (see 3.2 Digital Encoding of Characters) is known at all
times and places where text is exchanged or processed.
[S] Specifications MUST
either specify a unique encoding, or provide character encoding identification
mechanisms such that the encoding of text can always be reliably
identified. [S] When
designing a new protocol, format or API, specifications
SHOULD mandate a unique character
encoding.
Mandating a unique character encoding is simple, efficient, and
robust. There is no need for specifying, producing, transmitting, and
interpreting encoding tags. At the receiver, the encoding will always be
understood. There is also no ambiguity if data is transferred
non-electronically and later has to be converted back to a digital
representation. Even when there is a need for compatibility with existing data,
systems, protocols and applications, multiple encodings can often be dealt with
at the boundaries or outside a protocol, format, or API. The
DOM [DOM Level 1] is an
example of where this was done. The advantages of choosing a unique encoding
become more important the smaller the pieces of text used are and the closer to
actual processing the specification is.
[S] When a unique encoding is
mandated, the encoding MUST be UTF-8, UTF-16 or
UTF-32. [S] If a unique
encoding is mandated and compatibility with US-ASCII is desired, UTF-8 (see
[RFC 2279]) is RECOMMENDED. In
other situations, such as for APIs, UTF-16 or UTF-32 may be more appropriate.
Possible reasons for choosing one of these include efficiency of internal
processing and interoperability with other processes.
NOTE: The IETF Charset Policy [RFC 2277] specifies that
on the Internet "Protocols MUST be able to use the UTF-8
charset".
NOTE: The XML 1.0 specification [XML 1.0] requires all
conforming XML processors to accept both UTF-16 and UTF-8.
The MIME Internet specification [MIME] provides a
good example of a mechanism for character encoding identification. The MIME
charset parameter definition is intended to supply sufficient
information to uniquely decode the sequence of bytes of the received data into
a sequence of characters. The values are drawn from the IANA charset registry
[IANA].
NOTE: In practice there is wide variation among implementations, so
uniqueness cannot be depended upon. See the end of
3.5 Reference Processing Model for more information.
NOTE: The term 'charset' derives from 'character
set', an expression with a long and tortured history (see
[Connolly] for a discussion).
[S] Specifications
SHOULD avoid using the expression 'character
set', as well as the term 'charset' to refer to a character
encoding scheme, except when the latter is used to refer to the MIME
charset parameter or its IANA-registered values. The terms
'character encoding' or 'character encoding scheme'
are RECOMMENDED.
NOTE: In XML, the XML declaration or the text declaration contains a
pseudo-attribute called encoding which identifies the character
encoding using the IANA charset.
The IANA charset registry is the official list of names and
aliases for character encodings on the Internet.
[S] If the unique encoding
approach is not taken, specifications SHOULD mandate 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.
[S] The 'x-' convention for
unregistered character encoding names SHOULD NOT be used,
having led to abuse in the past. ('x-' was used
for character encodings that were widely used, even long after there was an
official registration.)
[I] [C] Content and software
that label textual data MUST use one of the names mandated
by the appropriate specification (e.g. the XML specification when editing XML
text) and SHOULD use the MIME preferred name of an encoding
to label data in that encoding.
[I] [C] An IANA-registered
charset name MUST NOT be used to label textual data
in an encoding other than the one identified in the IANA registration of that
name.
[S] 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
encodings and SHOULD choose at least one of UTF-8 or UTF-16
as mandated encoding forms (encoding forms that MUST be
supported by implementations of the specification).
[S] Specifications MAY
define either UTF-8 or UTF-16 as a default encoding form (or both if they
define suitable means of distinguishing them), but they MUST
NOT use any other character encoding as a default.
[S] Specifications MUST NOT
use heuristics to determine the encoding of data.
[I] Receiving
software MUST determine the encoding of data from available
information according to appropriate specifications.
[I] When an IANA-registered charset
name is recognized, receiving software MUST interpret the
received data according to the encoding associated with the name in the IANA
registry. [I] When no charset
is provided receiving software MUST adhere to the default
encoding(s) specified in the specification.
[I] Receiving software
MAY recognize as many encodings (names and aliases) as
appropriate. A field-upgradeable mechanism may be appropriate
for this purpose. Certain encodings are more or less associated with certain
languages (e.g. Shift-JIS with Japanese); trying to support a given language or
set of customers may mean that certain encodings have to be supported. The
encodings that need to be supported may change over time. This document does
not give any advice on which encoding may be appropriate or necessary for the
support of any given language.
[I] Software
MUST completely implement the mechanisms for character
encoding identification and SHOULD implement them in such a
way that they are easy to use (for instance in HTTP servers).
[I] On interfaces to other protocols, software
SHOULD support conversion between Unicode encoding forms as
well as any other necessary conversions.
[C] Content
MUST make use of available facilities for character encoding
identification by always indicating character encoding; where the facilities
offered for character encoding identification include defaults (e.g. in XML 1.0
[XML 1.0]), relying on such defaults is sufficient to satisfy this
identification requirement.
Because of the layered Web architecture (e.g. formats used over
protocols), there may be multiple and at times conflicting information about
character encoding. [S] Specifications
MUST define conflict-resolution mechanisms (e.g. priorities)
for cases where there is multiple or conflicting information about character
encoding.
[I] [C] Software and content
MUST carefully follow conflict-resolution mechanisms where
there is multiple or conflicting information about character
encoding.
Unicode designates certain ranges of code points for private use:
the Private Use Area (U+E000-F8FF) and planes 15 and 16 (U+F0000-FFFFD and
U+100000-10FFFD). These code points are guaranteed to never be allocated to
standard characters, and are available for use by private agreement between a
producer and a recipient. However, their use is strongly discouraged, since
private agreements do not scale on the Web. Code points from different private
agreements may collide, and a private agreement and therefore the meaning of
the code points can quickly get lost.
[S] Specifications MUST
NOT define any assignments of private use code
points. [S] Conformance to a
specification MUST NOT require the use of private use area
characters.
[S] Specifications SHOULD
NOT provide mechanisms for agreement on private use code points
between parties and MUST NOT require the use of such
mechanisms.
[S] [I] Specifications and
implementations SHOULD be designed in such a way as to not
disallow the use of private use code points by private
arrangement. As an example, XML does not disallow the use of
private use code points.
[S] Specifications
MAY define markup to allow the transmission of symbols not
in Unicode or to identify specific variants of Unicode
characters.
EXAMPLE: MathML (see [MathML2]
section
3.2.9) defines an element mglyph for mathematical symbols
not in Unicode.
EXAMPLE: SVG (see [SVG]
section
10.14) defines an element altglyph which allows the
identification of specific display variants of Unicode characters.
In text-based protocols or formats where characters can be either
part of character data or of markup (see 3.5 Reference Processing Model), it
is often the case that certain characters are designated as having certain
specific protocol/format functions in certain contexts (e.g.
'<' and '&' serve as markup delimiters in HTML
and XML). These syntax-significant characters cannot be used to represent
themselves in text data in the same way as all other characters do. Also, often
formats are represented in an encoding that does not allow to represent all
characters directly.
To express syntax-significant or unrepresentable characters, a
technique called escaping is used. This works by creating an
additional syntactic construct, defining additional characters or defining
character sequences that have special meaning. Escaping a character means
expressing it using such a construct, appropriate to the format or protocol in
which the character appears; expanding an escape (or
unescaping) means replacing it with the character that it
represents.
Certain guidelines apply to the way specifications define character
escapes. [S] The guidelines in this document
relating to the
definition of character escapes
MUST be followed when designing new W3C protocols and
formats and SHOULD be followed as much as possible when
revising existing protocols and formats.
-
[S] Specifications
MUST NOT invent a new escaping mechanism if an appropriate
one already exists.
-
[S] The number of different
ways to escape a character SHOULD be minimized (ideally to
one). [A well-known counter-example is that for historical
reasons, both HTML and XML have redundant decimal (&#ddddd;) and
hexadecimal (&#xhhhh;) escapes.]
-
[S] Explicit end delimiters
MUST be provided. Escapes such as \uABCD where the end
delimiter is a space or any character other than [01-9A-F]
SHOULD be avoided. These escapes are not
clear visually, and can cause an editor to insert spurious line-breaks when
word-wrapping on spaces. Forms like SPREAD's &UABCD; [SPREAD]or XML's &#xhhhh;, where the escape is explicitly terminated
by a semicolon, are much better.
-
[S] Whenever specifications
define escapes that allow the representation of characters using a number the
number SHOULD be in hexadecimal
notation.
-
[S] Escaped characters
SHOULD be acceptable wherever unescaped characters are; this
does not preclude that a syntax-significant character, when escaped, loses its
significance in the syntax. In particular, escaped characters
SHOULD be acceptable in identifiers and
comments.
Certain guidelines apply to content developers, as well as to
software that generates content:
-
[I] [C] Escapes
SHOULD be avoided when the characters to be expressed are
representable in the character encoding of the document.
-
[I] [C] Since
character set standards usually list character numbers as hexadecimal, content
SHOULD use the hexadecimal form of escapes when there is
one.
This chapter discusses character data normalization for the Web.
4.1 Motivation discusses the need for
normalization, and in particular early uniform normalization.
4.2 Definitions for W3C Text
Normalization defines full normalization and gives
examples. 4.3 Responsibility for
Normalization assigns reponsibilities
to various components and situations.
As explained at length in Requirements for String Identity
Matching and String Indexing [CharReq], the existence,
in many character encoding schemes, of multiple representations for what users
perceive as the same string makes it necessary to define character data
normalization. Without a precise specification, it is not possible to determine
reliably whether or not two strings are identical. Such a specification must
take into account character encoding, the way normalization is to be performed
and where or when (by sender or recipient) to perform it.
String identity is central to the correct functioning of much
software, and in particular of large parts of the Web infrastructure
(protocols, formats, etc.). Incorrect string matching can have far reaching
consequences, including the creation of security holes. Consider a contract,
encoded in XML, for buying goods: each item sold is described in an
artículo element; unfortunately, "artículo" is subject to
different representations in the character encoding of the contract. Suppose
that the contract is viewed and signed by means of a user agent that looks for
artículo elements, extracts them (matching on the element name),
presents them to the user and adds up their prices. If different instances of
the artículo element happen to be represented differently in a
particular contract, then the buyer and seller may see (and sign) different
contracts if their respective user agents perform string identity matching
differently, which is fairly likely in the absence of a well-defined
specification. The absence of a well-defined specification also means that
there is no way to resolve the ensuing contractual dispute.
The Unicode Consortium provides four standard normalization forms
(see Unicode Normalization Forms [UTR #15]).
For use on the Web, this document defines W3C Text Normalization by picking the
most appropriate of these (NFC) and additionally addressing the issues of
legacy encodings and of character escapes (which can denormalize text when
unescaped).
Roughly speaking, NFC is defined such that combining character
sequences (a base character followed by one or more combining characters) are
replaced, as far as possible, by canonically equivalent precomposed characters.
Text in a Unicode encoding form is said to be in NFC if it doesn't contain any
combining sequence that could be replaced and if any remaining combining
sequence is in canonical order.
This document also specifies that normalization is to be performed
early (by the sender) as opposed to late (by the
recipient). The reasons for that choice are manifold:
-
Almost all legacy data as well as data created by current
software is normalized (using NFC).
-
The number of Web components that generate or transform text
is considerably smaller than the number of components that receive text and
need to perform matching or other processes requiring normalized text.
-
Current receiving components (browsers, XML parsers, etc.)
implicitly assume early normalization by not performing normalization
themselves. This is a vast legacy.
-
Web components that generate and process text are in a much
better position to do normalization than other components; in particular, they
may be aware that they deal with a restricted repertoire only.
-
Not all components of the Web that implement functions such as
string matching can reasonably be expected to do normalization. This, in
particular, applies to very small components and components in the lower layers
of the architecture.
-
Forward-compatibility issues can be dealt with more easily:
less software needs to be updated, namely only the software that generates
newly introduced characters.
-
It improves matching in cases where the character encoding is
partly undefined, such as URIs [RFC 2396] in which non-ASCII bytes
have no defined meaning.
-
It is a prerequisite for comparison of encrypted strings (see
[CharReq],
section
2.7).
Text data is, for the purposes of this specification,
Unicode-normalized if it is in a
Unicode encoding form
and is in Unicode Normalization Form C (according to version 3.1.0
of [UTR #15]).
Text data is fully normalized if:
-
the data is Unicode-normalized and does not
contain any character escapes whose unescaping would cause the data to become
no longer Unicode-normalized; or
-
the data is in a legacy encoding and, if it
were transcoded to a Unicode encoding form by a
normalizing
transcoder, the resulting data would satisfy clause 1 above.
In the remainder of this specification, normalized is
used to mean 'fully normalized', unless otherwise indicated.
NOTE: A consequence of this definition is that legacy text (i.e. text
in a legacy encoding) is always normalized unless i) a normalizing transcoder
cannot exist for that encoding (e.g. because the repertoire contains characters
not in Unicode) or ii) the text contains escapes which, once expanded, result
in un-normalized text.
NOTE: Full normalization is specified against the context of a markup
language (or the absence thereof), which specifies the form of escapes. For
plain text (no escapes) in a Unicode encoding form, full normalization and
Unicode-normalization are equivalent.
The string 'suçon', expressed as the sequence of five
characters U+0073 U+0075 U+00E7 U+006F U+006E and encoded in a Unicode encoding
form, is both Unicode-normalized and fully normalized. The same string encoded
in a legacy encoding for which there exists a normalizing transcoder would be
fully normalized but not Unicode-normalized (since not in a Unicode encoding
form).
In an XML or HTML context, the string suçon is also both fully normalized and, if encoded in a Unicode encoding
form, Unicode-normalized. Expanding ç yields suçon as above, which contains no replaceable combining sequence.
The string 'suçon', expressed as the sequence of
six characters U+0073 U+0075 U+0063 U+0327 U+006F
U+006E (U+0327 is the COMBINING CEDILLA) and encoded in a
Unicode encoding form, is neither Unicode-normalized (since the combining
sequence U+0063 U+0327 is replaceable by the precomposed U+00E7
'ç') nor fully normalized (since in a Unicode encoding form but
not Unicode-normalized).
In an XML or HTML context, the string suçon is not fully normalized, regardless of encoding form, because expanding
̧ yields the sequence suc¸on which is not Unicode-normalized ('c¸' is replaceable by
'ç'). Unicode-normalization, however, is defined only for plain
text, doesn't know that ̧ represents a character in XML or HTML and
considers it just a sequence of characters. Therefore, the string suçon in a Unicode encoding form is Unicode-normalized since it
contains no replaceable combining sequence. (The latter example does not imply
that Unicode-normalization is sufficient to meet the normalization requirements
of the Web; it just illustrates a case where Unicode-normalization and full
normalization differ).
The string <elem>/ foobar</elem>, where the '/' immediately after <elem> stands for the character U+0338 COMBINING LONG SOLIDUS
OVERLAY, is neither Unicode-normalized nor fully normalized, since the
U+0338 '/' combines with the '>' (yielding U+226F
NOT GREATER-THAN).
NOTE: From this example, it follows that it is impossible to produce a
normalized XML or HTML document containing the character U+0338
COMBINING LONG SOLIDUS OVERLAY immediately following an element
tag, comment, CDATA section or processing instruction. It is noteworthy that
U+0338 COMBINING LONG SOLIDUS OVERLAY also combines with
'<', yielding U+226E NOT LESS-THAN.
Consequently, U+0338 COMBINING LONG SOLIDUS OVERLAY should
remain excluded from XML identifiers.
This section defines the responsibilities for normalization for
various components and situations, based on the goal of early uniform
normalization.
[C] All content on the Web
MUST be fully normalized.
[I] Producers
MUST produce text data in normalized form, unless they are
willing to accept the consequences (loss of integrity and security, high
probability of rejection by recipients) of un-normalized data.
For the purpose of W3C specifications and their implementations, the producer
of text data is the sender of the data in the case of protocols and the tool
that produces the data in the case of formats.
NOTE: As an optimization, it is perfectly acceptable for a
system to define the producer to be the actual producer (e.g. a
small device) together with a remote component (e.g. a server serving as a kind
of proxy) to which normalization is delegated. In such a case, the
communications channel between the device and proxy server is considered to be
internal to the system, not part of the Web. Only data normalized
by the proxy server is to be exposed to the Web at large, as shown in the
illustration below:
NOTE: Normalization is the responsibility of the producer as a whole.
This specification does not assign responsibility for normalization to any
particular component of the producer (for instance a DOM implementation).
NOTE: Implementers of producer software are encouraged to delegate
normalization to their respective data sources wherever possible. Examples of
data sources are operating systems, libraries, and keyboard drivers. One way of
ensuring that user input results in normalized data is to not provide any way
of creating denormalized data.
[I] The recipients of text data
MUST verify the normalization of data they receive and
reject un-normalized data, unless they are willing to accept the consequences
(loss of integrity and security) of un-normalized data.
[I] Recipients MUST NOT
normalize the data that they receive.
[I] Recipients which transcode text data from
a legacy encoding to a Unicode encoding form MUST use a
normalizing
transcoder.
NOTE: The prohibition of normalization by recipients is necessary to
avoid the security issues mentioned in section
4.1 Motivation.
[I] When a recipient returns
un-normalized text to a sender (e.g. to indicate an error or fault), that
recipient MAY return the text without normalizing
it.
[I] If a software module functions
as both a producer and a recipient of text data (e.g. a browser/editor),
normalization MUST be applied in the producer part but
MUST NOT be applied in the recipient
part.
[I] Intermediate
(recipient/producer) components whose role involves modification of text data
MUST ensure that their modifications do not result in
denormalization of any data exposed (sent on the network, saved to disk,
returned in an API call, etc.).
NOTE: Consequently, an intermediate component, in a system that packages
a payload in some control information, may modify the control information
without having to renormalize the payload.
[I] Software
MUST behave as if normalization took place
after each modification, so that any subsequent matching, indexing or other
normalization-sensitive operations always behave as if they were
dealing with normalized data.
EXAMPLE: If the
'z' is deleted from the (normalized) string cz¸ (where '¸' represents a combining cedilla, U+0327),
normalization is necessary to turn the denormalized result c¸ into the properly normalized ç. Analogous cases exist for insertion and concatenation. If the software
that deletes the 'z' later uses the string in a
normalization-sensitive operation, it needs to normalize the string before this
operation to ensure correctness; otherwise, normalization may be deferred until
the data is exposed.
[I] Intermediate components whose
role does not involve modification of the data (e.g. caching proxies)
MUST NOT reject un-normalized data and MUST
NOT perform normalization.
[S] In specifications of markup
languages, syntax-significant characters MUST be chosen that
do not combine with any other characters in NFC. This is to
avoid problems such as U+0338 COMBINING LONG SOLIDUS OVERLAY
combining with the '<' and '>' delimiters in
XML (see the last example in section 4.2.3 Examples above).
This specification does not address the suitability of particular
characters for use in markup languages, in particular formatting characters and
compatibility equivalents. For detailed recommendations about the use of
compatibility and formatting characters, see Unicode in XML and other
Markup Languages [UXML].
[S] Specifications
SHOULD exclude compatibility characters in the syntactic
elements (markup, delimiters, identifiers) of the formats they
define.
One important operation that depends on early normalization is
string identity matching [CharReq], which is a
subset of the more general problem of string matching. There are various
degrees of specificity for string matching, from approximate matching such as
regular expressions or phonetic matching, to more specific matches such as
case-insensitive or accent-insensitive matching and finally to identity
matching. In the Web environment, where multiple encodings are used to
represent strings, including some encodings which allow multiple
representations for the same thing, identity is defined to occur
if and only if the compared strings contain no user-identifiable distinctions.
This definition is such that strings do not match when they differ in case or
accentuation, but do match when they differ only in non-semantically
significant ways such as encoding, use of escapes (of potentially different
kinds), or use of precomposed vs. decomposed character sequences.
To avoid unnecessary conversions and, more importantly,
to ensure predictability and correctness, it is necessary for all components of
the Web to use the same identity testing mechanism. Conformance to the rule
that follows meets this requirement and supports the above definition of
identity. [S] [I] String
identity matching MUST be performed as if the following
steps were followed:
-
Early uniform normalization to fully normalized form, as defined
in 4.2.2 Fully Normalized Text. In accordance with section
4 Early Uniform Normalization, this step MUST be
performed by the producers of the strings to be compared.
-
Conversion to a common encoding of UCS, if necessary.
-
Expansion of all escapes.
-
Testing for bit-by-bit identity.
Step 1 ensures 1) that the identity matching process can produce
correct results using the next three steps and 2) that a minimum of effort is
spent on solving the problem.
[S] [I] Forms of
string matching other than identity SHOULD be based on the
steps specified in this document for
string identity matching. Taking into account normalization
and escapes is necessary so that, for example, a case-insensitive match of suçon against sucçon or against SUC¸ON returns TRUE.
NOTE: The expansion of escapes (step 3 above) is dependent on context,
i.e. on which markup or programming language is considered to apply when the
string matching operation is performed. Consider a search for the string
'suçon' in an XML document containing sucçon but not suçon. If the search is performed in a plain text editor, the context is
plain text (no markup or programming language applies), the
ç escape is not recognized, hence not expanded and the search fails.
If the search is performed in an XML browser, the context is XML,
the escape (defined by XML) is expanded and the search succeeds.
An intermediate case would be an XML editor that
purposefully provides a view of an XML document with entity
references left unexpanded. In that case, a search over that pseudo-XML view
will deliberately not expand entities: in that particular context,
entity references are not considered escapes and need not be expanded.
There are many situations where a software process needs to access a
substring or to point within a string and does so by the use of
indices, i.e. numeric "positions" within a string.
Where such indices are exchanged between components of the Web, there is a need
for an agreed-upon definition of string indexing in order to ensure consistent
behavior. The requirements for string indexing are discussed in
Requirements for String Identity Matching
[CharReq],
section 4. The two
main questions that arise are: "What is the unit of counting?" and
"Do we start counting at 0 or 1?".
Depending on the particular requirements of a process, the unit of
counting may correspond to any of the definitions of a string provided in
section 3.4 Strings. In particular:
-
[S] [I] The
character string is
RECOMMENDED as a basis for string indexing.
(Example: the XML Path Language [XPath]).
-
[S] [I] 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. (Example: the use of
UTF-16 in [DOM Level 1]).
Counting graphemes will
become a good option where user interaction is the primary concern, once a
suitable definition is widely accepted.
It is noteworthy that there exist other, non-numeric ways of
identifying substrings which have favorable properties. For instance,
substrings based on string matching are quite robust against small edits;
substrings based on document structure (in structured formats such as XML) are
even more robust against edits and even against translation of a document from
one human language to another.
[S] Specifications that need a way to identify
substrings or point within a string SHOULD provide ways
other than string indexing to perform this operation.
[I] [C] Users of
specifications (software developers, content developers)
SHOULD whenever possible prefer ways other than string
indexing to identify substrings or point within a string.
Experience shows that more general, flexible and robust specifications
result when individual characters are understood and processed as substrings,
identified by a position before and a position after the substring.
Understanding indices as boundary positions between the counting
units also makes it easier to relate the indices resulting from the different
string definitions. [S] 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.
[S] Specifications of APIs
SHOULD NOT specify single character or single encoding-unit
arguments.
EXAMPLE: uppercase('ß') cannot return the proper result (the two-character string
'SS') if the return type of the uppercase
function is defined to be a single character.
The issue of index origin, i.e. whether we count from 0 or 1, actually
arises only after a decision has been made on whether it is the units
themselves that are counted or the positions between the units.
[S] 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.
According to the definition in RFC 2396 [RFC 2396], URI
references are restricted to a subset of US-ASCII, with an escaping mechanism
to encode arbitrary byte values, using the %HH convention. However, the %HH
convention by itself is of limited use because there is no definitive mapping
from characters to bytes. Also, non-ASCII characters cannot be used directly.
Internationalized Resource Identifiers (IRI)
[I-D URI-I18N] solves both problems with an uniform approach that
conforms to the
Reference Processing Model.
[S] W3C specifications that define
protocol or format elements (e.g. HTTP headers, XML attributes, etc.) which are
to be interpreted as URI references (or specific subsets of URI references,
such as absolute URI references, URIs, etc.) MUST use
Internationalized Resource Identifiers (IRI)
[I-D URI-I18N] (or an appropriate subset thereof).
[S] W3C specifications MUST
define when the conversion from IRI references to URI references (or subsets
thereof) takes place, in accordance with Internationalized Resource
Identifiers (IRI) [I-D URI-I18N].
[S] W3C specifications that define
new syntax for URIs, such as a new URI scheme or a new kind of fragment
identifier, MUST specify that characters outside the
US-ASCII repertoire are encoded using UTF-8 and %HH-escaping, in accordance
with Guidelines for new URL Schemes [RFC 2718], Section 2.2.5. This will make sure that these
schemes or fragment identifiers can be used in IRIs in the natural way.
Specifications often need to make references to the Unicode standard
or International Standard ISO/IEC 10646. Such references must be made with
care, especially when normative. The questions to be considered are:
-
Which standard should be referenced?
-
How to reference a particular version?
-
When to use versioned vs unversioned references?
ISO/IEC 10646 is developed and published jointly by
ISO (the International
Organisation for Standardisation) and
IEC (the International
Electrotechnical Commission). The Unicode Standard is developed and published
by the
Unicode Consortium, an
organization of major computer corporations, software producers, database
vendors, national governments, research institutions, international agencies,
various user groups, and interested individuals. The Unicode Standard is
comparable in standing to W3C Recommendations.
ISO/IEC 10646 and Unicode define exactly the same CCS (same
repertoire, same code points) and encoding forms. They are actively maintained
in synchrony by liaisons and overlapping membership between the respective
technical committees. In addition to the jointly defined CCS and encoding
forms, the Unicode Standard adds normative and informative lists of character
properties, normative character equivalence and normalization specifications, a
normative algorithm for bidirectional text and a large amount of useful
implementation information. In short, Unicode adds semantics to the characters
that ISO/IEC 10646 merely enumerates. Conformance to Unicode implies
conformance to ISO/IEC 10646, see [Unicode 3.0] Appendix C.
[S] 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. By providing a reference to The Unicode Standard
implementers can benefit from the wealth of information provided in the
standard and on the Unicode Consortium Web site.
The fact that both ISO/IEC 10646 and Unicode are evolving (in
synchrony) raises the issue of versioning: should a specification refer to a
specific version of the standard, or should it make a generic reference, so
that the normative reference is to the version current at the time of
reading the specification? In general the answer is
both. [S] 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 (an example would
be the set of characters acceptable as Name characters in XML 1.0
[XML 1.0], which is an enumerated list that parsers must implement
to validate names).
A generic reference can be formulated in two ways:
-
By explicitly including a generic entry in the
bibliography section of a specification and simply referring to that entry in
the body of the specification. Such a generic entry contains text such as
"... as it may from time to time be revised or amended".
-
By including a specific entry in the bibliography
and adding text such as "... as it may from time to time be revised or
amended" at the point of reference in the body of the specification.
It is an editorial matter, best left to each specification, which of
these two formulations is used. Examples of the first formulation can be found
in the bibliography of this specification (see the entries for
[ISO/IEC 10646] and [Unicode]). Examples of the latter,
as well as a discussion of the versioning issue with respect to MIME
charset parameters for UCS encodings, can be found in
[RFC 2279] and [RFC 2781].
[S] All generic
references to Unicode MUST refer to Unicode 3.0[Unicode 3.0] or later.
[S] Generic references to ISO/IEC 10646
MUST be written such that they make allowance for the future
publication of additional parts of the standard. They
MUST refer to ISO/IEC 10646-1:2000
[ISO/IEC 10646-1:2000] or later, including any
amendments.
A few examples will help make sense all this complexity
of text in computers (which is mostly a reflection of the complexity of human
writing systems). Let us start with a very simple example: a user, equipped
with a US-English keyboard, types "Foo", which the computer
encodes as 16-bit values (the UTF-16 encoding of Unicode) and displays on the
screen.
The only complexity here is the use of a modifier (Shift) to input the
capital 'F'.
A slightly more complex example is a user typing 'çé' on
a traditional French-Canadian keyboard, which the computer again encodes in
UTF-16 and displays. We assume that this particular computer uses a fully
composed form of UTF-16.
A few interesting things are happening here: when the user types the
cedilla ('¸'), nothing happens except for a change of state of the
keyboard driver; the cedilla is a dead key. When the driver gets
the c keystroke, it provides a complete 'ç' character to the
system, which represents it as a single 16-bit code unit and displays a
'ç' glyph. The user then presses the dedicated 'é'
key, which results in, again, a character represented by two bytes. Most
systems will display this as one glyph, but it is also possible to combine two
glyphs (the base letter and the accent) to obtain the same rendering.
On to a Japanese example: our user employs a romaji input
method to type "
", which the
computer encodes in UTF-16 and displays.
The interesting aspect here is input: the user types Latin characters,
which are converted on the fly to kana (not shown here), and then to kanji when
the user requests conversion by pressing <space>; the kanji characters
are finally sent to the application when the user presses <return>. The
user has to type a total of nine keystrokes before the three characters are
produced, which are then encoded and displayed rather trivially.
An Arabic example will show different phenomena:
Here the first two keystrokes each produce an input character and an
encoded character, but the pair is displayed as a single glyph
('
', a lam-alef ligature). The next keystroke
is a lam-alef, which some Arabic keyboards have; it produces the same two
characters which are displayed similarly, but this second lam-alef is placed to
the left of the first one when displayed. The last two keystrokes
produce two identical characters which are rendered by two different glyphs (a
medial form followed to its left by a final form). We thus have 5 keystrokes
producing 6 characters and 4 glyphs laid out right-to-left.
A final example in Tamil, typed with an ISCII
keyboard, will illustrate some additional phenomena:
Here input is straightforward, but note that contrary to the preceding
accented Latin example, the diacritic '
' (virama, vowel killer) is entered
after the '
' to which it applies. Rendering is
interesting for the last two characters. The last one ('
')
clearly consists of two glyphs which surround the glyph of the
next to last character ('
').
A number of operations routinely performed on
text can be impacted by the complexities of the world's writing systems. An
example is the operation of selecting text on screen by a pointing device in a
bidirectional (bidi) context (see 3.1.3 Units of Visual
Rendering).
Let's have a look at some bidi text, in this case Arabic letters (written
right-to-left) mixed with Arabic-Hindi digits (left-to-right):
Special thanks go to Ian Jacobs for ample help with editing. Tim
Berners-Lee and James Clark provided important details in the section on URIs.
The W3C I18N WG and IG, as well as others, provided many comments and
suggestions.
- IANA
- Internet Assigned Numbers Authority,
Official
Names for Character Sets. (See
ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets.)
- ISO/IEC 10646
- ISO/IEC 10646-1:2000,
Information
technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1:
Architecture and Basic Multilingual Plane, as, from time to time,
amended, replaced by a new edition or expanded by the addition of new parts.
(See
http://www.iso.ch/ for the
latest version.)
- ISO/IEC 10646-1:2000
- ISO/IEC
10646-1:2000,
Information
technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1:
Architecture and Basic Multilingual Plane. (See
http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=29819.)
- MIME
- Multipurpose Internet Mail
Extensions (MIME). Part One: Format of Internet Message Bodies, N.
Freed, N. Borenstein, RFC 2045, November 1996,
http://www.ietf.org/rfc/rfc2045.txt.
Part Two: Media Types, N. Freed, N. Borenstein, RFC 2046,
November 1996. Part Three: Message Header Extensions for Non-ASCII
Text, K. Moore, RFC 2047, November 1996. Part Four:
Registration Procedures, N. Freed, J. Klensin, J. Postel, RFC 2048,
November 1996. Part Five: Conformance Criteria and
Examples, N. Freed, N. Borenstein, RFC 2049, November 1996.
- RFC 2070
- F. Yergeau, G. Nicol, G. Adams, M.
Dürst, Internationalization of the
Hypertext Markup Language, IETF RFC 2070, January 1997. (See
http://www.ietf.org/rfc/rfc2070.txt.)
- RFC 2119
- S. Bradner,
Key words for use in RFCs
to Indicate Requirement Levels, IETF RFC 2119. (See
http://www.ietf.org/rfc/rfc2119.txt.)
- RFC 2396
- T. Berners-Lee, R. Fielding, L.
Masinter, Uniform Resource
Identifiers (URI): Generic Syntax, IETF RFC 2396, August 1998. (See
http://www.ietf.org/rfc/rfc2396.txt.)
- RFC 2732
- R. Hinden, B. Carpenter, L.
Masinter, Format for
Literal IPv6 Addresses in URL's, IETF RFC 2732, 1999. (See
http://www.ietf.org/rfc/rfc2732.txt.)
- Unicode
- The Unicode Consortium,
The Unicode Standard -- Version 3.0, ISBN 0-201-61633-5,
as updated from time to time by the publication of new versions. (See
http://www.unicode.org/unicode/standard/versions/
for the latest version and additional information on versions of the standard
and of the Unicode Character Database).
- Unicode 3.0
- The Unicode Consortium,
The Unicode Standard -- Version 3.0, ISBN 0-201-61633-5.
(See
http://www.unicode.org/unicode/standard/versions/Unicode3.0.html.)
- UTR #15
- Mark Davis, Martin Dürst,
Unicode
Normalization Forms, Unicode Standard Annex #15. (See
http://www.unicode.org/unicode/reports/tr15
for the latest version).
Version
3.1.0 (March 2001) is at
http://www.unicode.org/unicode/reports/tr15/tr15-21.html.
- CharReq
- Martin J. Dürst,
Requirements for String
Identity and Character Indexing Definitions for the WWW, W3C Working
Draft. (See
http://www.w3.org/TR/WD-charreq.)
- Connolly
- D. Connolly,
Character
Set Considered Harmful, W3C Note. (See
http://www.w3.org/MarkUp/html-spec/charset-harmful.)
- CSS2
- Bert Bos, Håkon Wium Lie, Chris Lilley,
Ian Jacobs, Eds., Cascading
Style Sheets, level 2 (CSS2 Specification), W3C Recommendation. (See
http://www.w3.org/TR/REC-CSS2.)
- DOM Level 1
- Vidur Apparao et al.,
Document Object Model
(DOM) Level 1 Specification, W3C Recommendation. (See
http://www.w3.org/TR/REC-DOM-Level-1/.)
- HTML 4.0
- Dave Raggett, Arnaud Le Hors, Ian
Jacobs, Eds., HTML 4.0
Specification, W3C Recommendation, 18-Dec-1997 (See
http://www.w3.org/TR/REC-html40-971218/.)
- HTML 4.01
- Dave Raggett, Arnaud Le Hors, Ian
Jacobs, Eds., HTML 4.01
Specification, W3C Recommendation, 24-Dec-1999. (See
http://www.w3.org/TR/html401/.)
- I-D URI-I18N
- Larry Masinter, Martin Dürst,
Internationalized
Resource Identifiers (IRI), Internet-Draft, November 2001. (See
http://www.w3.org/International/2001/draft-masinter-url-i18n-08.txt.)
- Info URI-I18N
- Internationalization:
URIs and other identifiers. (See
http://www.w3.org/International/O-URL-and-ident.)
- ISO/IEC 9541-1
- ISO/IEC 9541-1:1991,
Information
technology -- Font information interchange -- Part 1: Architecture. (See
http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=17277
for the latest version.)
- MathML2
- David Carlisle, Patrick Ion, Robert
Miner, Nico Poppelier, Eds., Mathematical Markup Language (MathML)
Version 2.0, W3C Recommendation, 21 February 2001. (See
http://www.w3.org/TR/MathML2/.)
- Nicol
- Gavin Nicol,
The
Multilingual World Wide Web, Chapter 2: The WWW As A Multilingual
Application. (See
http://www.mind-to-mind.com/i18n/articles/multilingual/multilingual-www.html.)
- RFC 2070
- F. Yergeau, G. Nicol, G. Adams, M.
Dürst, Internationalization of the
Hypertext Markup Language, IETF RFC 2070, January 1997. (See
http://www.ietf.org/rfc/rfc2070.txt.)
- RFC 2277
- H. Alvestrand,
IETF Policy on Character
Sets and Languages, IETF RFC 2277, BCP 18, January 1998. (See
http://www.ietf.org/rfc/rfc2277.txt.)
- RFC 2279
- F. Yergeau,
UTF-8, a transformation
format of ISO 10646, IETF RFC 2279, January 1998. (See
http://www.ietf.org/rfc/rfc2279.txt.)
- RFC 2718
- L. Masinter, H. Alvestrand, D.
Zigmond, R. Petke, Guidelines for new URL
Schemes, IETF RFC 2718, November 1999. (See
http://www.ietf.org/rfc/rfc2718.txt.)
- RFC 2781
- P. Hoffman, F. Yergeau,
UTF-16, an encoding of ISO
10646, IETF RFC 2781, February 2000. (See
http://www.ietf.org/rfc/rfc2781.txt.)
- SPREAD
- SPREAD -
Standardization Project for East Asian Documents Universal Public Entity
Set. (See
http://www.ascc.net/xml/resource/entities/index.html)
- SVG
- Jon Ferraiolo, Ed.,
Scalable Vector Graphics (SVG) 1.0
Specification, W3C Recommendation, 4 September 2001. (See
http://www.w3.org/TR/SVG/.)
- UTR #17
- Ken Whistler, Mark Davis,
Character
Encoding Model, Unicode Technical Report #17. (See
http://www.unicode.org/unicode/reports/tr17/.)
- UXML
- Martin Dürst and Asmus Freytag,
Unicode in XML and other
Markup Languages, Unicode Technical Report #20 and W3C Note. (See
http://www.w3.org/TR/unicode-xml.)
- XLink
- Steve DeRose, Eve Maler, David Orchard,
Eds, XML Linking Language (XLink)
Version 1.0, W3C Recommendation, 27 June 2001. (See
http://www.w3.org/TR/xlink/.)
- XML 1.0
- Tim Bray, Jean Paoli, C. M.
Sperberg-McQueen, Eve Maler, Eds.,
Extensible Markup Language (XML)
1.0, W3C Recommendation. (See
http://www.w3.org/TR/REC-xml.)
- XML Schema-2
- Paul V. Biron , Ashok
Malhotra , Eds., XML Schema
Part 2: Datatypes, W3C Recommendation. (See
http://www.w3.org/TR/xmlschema-2.)
- XML Japanese Profile
- MURATA
Makoto Ed., XML Japanese
Profile, W3C Note. (See
http://www.w3.org/TR/japanese-xml/.)
- XPath
- James Clark, Steve DeRose, Eds,
XML Path Language (XPath) Version
1.0, W3C Recommendation, 16 November 1999. (See
http://www.w3.org/TR/xpath.)
- XPointer
- Steve DeRose, Eve Maler, Ron
Daniel Jr., Eds, XML Pointer Language
(XPointer) Version 1.0, W3C Candidate Recommendation, 11 September
2001. (See http://www.w3.org/TR/2001/CR-xptr-20010911/.)
Normalization: changed from "recipients MUST
NOT normalize" to "recipients MUST check and
reject un-normalized data".
Clarified conformance model, in particular introduced [S][I][C]
specifiers for requirements.
Made numerous other changes listed in
Character
Model for the World Wide Web 1.0 Last Call Comments (Members
only).
Fixed countless typos and unclear/ambiguous sentences.
Updated references.
