RFC 2070
Network Working Group F. Yergeau
Request for Comments: 2070 Alis Technologies
Category: Standards Track G. Nicol
Electronic Book Technologies
G. Adams
Spyglass
M. Duerst
University of Zurich
January 1997
Internationalization of the Hypertext Markup Language
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Hypertext Markup Language (HTML) is a markup language used to
create hypertext documents that are platform independent. Initially,
the application of HTML on the World Wide Web was seriously
restricted by its reliance on the ISO-8859-1 coded character set,
which is appropriate only for Western European languages. Despite
this restriction, HTML has been widely used with other languages,
using other coded character sets or character encodings, at the
expense of interoperability.
This document is meant to address the issue of the
internationalization (i18n, i followed by 18 letters followed by n)
of HTML by extending the specification of HTML and giving additional
recommendations for proper internationalization support. A foremost
consideration is to make sure that HTML remains a valid application
of SGML, while enabling its use with all languages of the world.
Table of Contents
1. Introduction .................................................. 2
1.1. Scope ...................................................... 2
1.2. Conformance ................................................ 3
2. The document character set ..................................... 4
2.1. Reference processing model ................................. 4
2.2. The document character set ................................. 6
2.3. Undisplayable characters ................................... 8
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3. The LANG attribute.............................................. 8
4. Additional entities, attributes and elements ................... 9
4.1. Full Latin-1 entity set .................................... 9
4.2. Markup for language-dependent presentation ................ 10
5. Forms ..........................................................16
5.1. DTD additions ..............................................16
5.2. Form submission ............................................17
6. External character encoding issues .............................18
7. HTML public text ...............................................20
7.1. HTML DTD ...................................................20
7.2. SGML declaration for HTML ..................................35
7.3. ISO Latin 1 character entity set ...........................37
8. Security Considerations.........................................40
Bibliography ......................................................40
Authors' Addresses ................................................43
1. Introduction
The Hypertext Markup Language (HTML) is a markup language used to
create hypertext documents that are platform independent. Initially,
the application of HTML on the World Wide Web was seriously
restricted by its reliance on the ISO-8859-1 coded character set,
which is appropriate only for Western European languages. Despite
this restriction, HTML has been widely used with other languages,
using other coded character sets or character encodings, through
various ad hoc extensions to the language [TAKADA].
This document is meant to address the issue of the
internationalization of HTML by extending the specification of HTML
and giving additional recommendations for proper internationalization
support. It is in good part based on a paper by one of the authors
on multilingualism on the WWW [NICOL]. A foremost consideration is
to make sure that HTML remains a valid application of SGML, while
enabling its use with all languages of the world.
The specific issues addressed are the SGML document character set to
be used for HTML, the proper treatment of the charset parameter
associated with the "text/html" content type and the specification of
some additional elements and entities.
1.1 Scope
HTML has been in use by the World-Wide Web (WWW) global information
initiative since 1990. This specification extends the capabilities
of HTML 2.0 (RFC 1866), primarily by removing the restriction to the
ISO-8859-1 coded character set [ISO-8859].
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HTML is an application of ISO Standard 8879:1986, Information
Processing Text and Office Systems -- Standard Generalized Markup
Language (SGML) [ISO-8879]. The HTML Document Type Definition (DTD)
is a formal definition of the HTML syntax in terms of SGML. This
specification amends the DTD of HTML 2.0 in order to make it
applicable to documents encompassing a character repertoire much
larger than that of ISO-8859-1, while still remaining SGML
conformant.
Both formal and actual development of HTML are advancing very fast.
The features described in this document are designed so that they can
(and should) be added to other forms of HTML besides that described
in RFC 1866. Where indicated, attributes introduced here should be
extended to the appropriate elements.
1.2 Conformance
This specification changes slightly the conformance requirements of
HTML documents and HTML user agents.
1.2.1 Documents
All HTML 2.0 conforming documents remain conforming with this
specification. However, the extensions introduced here make valid
certain documents that would not be HTML 2.0 conforming, in
particular those containing characters or character references
outside of the repertoire of ISO 8859-1, and those containing markup
introduced herein.
1.2.2. User agents
In addition to the requirements of RFC 1866, the following
requirements are placed on HTML user agents.
To ensure interoperability and proper support for at least ISO-
8859-1 in an environment where character encoding schemes other
than ISO-8859-1 are present, user agents MUST correctly interpret
the charset parameter accompanying an HTML document received from
the network.
Furthermore, conforming user-agents MUST at least parse correctly
all numeric character references within the range of ISO 10646-1
[ISO-10646].
Conforming user-agents are required to apply the BIDI presentation
algorithm if they display right-to-left characters. If there is
no displayable right-to-left character in a document, there is no
need to apply BIDI processing.
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2. The document character set
2.1. Reference processing model
This overview explains a reference processing model used for HTML,
and in particular the SGML concept of a document character set. An
actual implementation may widely differ in its internal workings from
the model given below, but should behave as described to an outside
observer.
Because there are various widely differing encodings of text, SGML
does not directly address how the sequence of characters that
constitutes an SGML document in the abstract sense are encoded by
means of a sequence of octets (or occasionally bit groups of another
length than 8) in a concrete realization of the document such as a
computer file. This encoding is called the external character
encoding of the concrete SGML document, and it should be carefully
distinguished from the document character set of the abstract HTML
document. SGML views the characters as a single set (called a
"character repertoire"), and a "code set" that assigns an integer
number (known as "character number") to each character in the
repertoire. The document character set declaration defines what each
of the character numbers represents [GOLD90, p. 451]. In most cases,
an SGML DTD and all documents that refer to it have a single document
character set, and all markup and data characters are part of this
set.
HTML, as an application of SGML, does not directly address the
question of the external character encoding. This is deferred to
mechanisms external to HTML, such as MIME as used by the HTTP
protocol or by electronic mail.
For the HTTP protocol [RFC2068], the external character encoding is
indicated by the "charset" parameter of the "Content-Type" field of
the header of an HTTP response. For example, to indicate that the
transmitted document is encoded in the "JUNET" encoding of Japanese
[RFC1468], the header will contain the following line:
Content-Type: text/html; charset=ISO-2022-JP
The term "charset" in MIME is used to designate a character encoding,
rather than merely a coded character set as the term may suggest. A
character encoding is a mapping (possibly many-to-one) of sequences
of octets to sequences of characters taken from one or more character
repertoires.
The HTTP protocol also defines a mechanism for the client to specify
the character encodings it can accept. Clients and servers are
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strongly requested to use these mechanisms to assure correct
transmission and interpretation of any document. Provisions that can
be taken to help correct interpretation, even in cases where a server
or client do not yet use these mechanisms, are described in section
6.
Similarly, if HTML documents are transferred by electronic mail, the
external character encoding is defined by the "charset" parameter of
the "Content-Type" MIME header field [RFC2045], and defaults to US-
ASCII in its absence.
No mechanisms are currently standardized for indicating the external
character encoding of HTML documents transferred by FTP or accessed
in distributed file systems.
In the case any other way of transferring and storing HTML documents
are defined or become popular, it is advised that similar provisions
be made to clearly identify the character encoding used and/or to use
a single/default encoding capable of representing the widest range of
characters used in an international context.
Whatever the external character encoding may be, the reference
processing model translates it to the document character set
specified in Section 2.2 before processing specific to SGML/HTML.
The reference processing model can be depicted as follows:
[resource]->[decoder]->[entity ]->[ SGML ]->[application]->[display]
[manager] [parser]
^ |
| |
+----------+
The decoder is responsible for decoding the external representation
of the resource to the document character set. The entity manager,
the parser, and the application deal only with characters of the
document character set. A display-oriented part of the application
or the display machinery itself may again convert characters
represented in the document character set to some other
representation more suitable for their purpose. In any case, the
entity manager, the parser, and the application, as far as character
semantics are concerned, are using the HTML document character set
only.
An actual implementation may choose, or not, to translate the
document into some encoding of the document character set as
described above; the behaviour described by this reference processing
model can be achieved otherwise. This subject is well out of the
scope of this specification, however, and the reader is invited to
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consult the SGML standard [ISO-8879] or an SGML handbook [BRYAN88]
[GOLD90] [VANH90] [SQ91] for further information.
The most important consequence of this reference processing model is
that numeric character references are always resolved with respect to
the fixed document character set, and thus to the same characters,
whatever the external encoding actually used. For an example, see
Section 2.2.
2.2. The document character set
The document character set, in the SGML sense, is the Universal
Character Set (UCS) of ISO 10646:1993 [ISO-10646], as amended.
Currently, this is code-by-code identical with the Unicode standard,
version 1.1 [UNICODE].
NOTE -- implementers should be aware that ISO 10646 is amended
from time to time; 4 amendments have been adopted since the
initial 1993 publication, none of which significantly affects this
specification. A fifth amendment, now under consideration, will
introduce incompatible changes to the standard: 6556 Korean Hangul
syllables allocated between code positions 3400 and 4DFF
(hexadecimal) will be moved to new positions (and 4516 new
syllables added), thus making references to the old positions
invalid. Since the Unicode consortium has already adopted a
corresponding amendment for inclusion in the forthcoming Unicode
2.0, adoption of DAM 5 is considered likely and implementers
should probably consider the old code positions as already
invalid. Despite this one-time change, the relevant standard
bodies have committed themselves not to change any allocated code
position in the future. To encode Korean Hangul irrespective of
these changes, the conjoining Hangul Jamo in the range 1110-11F9
can be used.
The adoption of this document character set implies a change in the
SGML declaration specified in the HTML 2.0 specification (section 9.5
of [RFC1866]). The change amounts to removing the first BASESET
specification and its accompanying DESCSET declaration, replacing
them with the following declaration:
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BASESET "ISO Registration Number 177//CHARSET
ISO/IEC 10646-1:1993 UCS-4 with implementation level 3
//ESC 2/5 2/15 4/6"
DESCSET 0 9 UNUSED
9 2 9
11 2 UNUSED
13 1 13
14 18 UNUSED
32 95 32
127 1 UNUSED
128 32 UNUSED
160 2147483486 160
Making the UCS the document character set does not create non-
conformance of any expression, construct or document that is
conforming to HTML 2.0. It does make conforming certain constructs
that are not admissible in HTML 2.0. One consequence is that data
characters outside the repertoire of ISO-8859-1, but within that of
UCS-4 become valid SGML characters. Another is that the upper limit
of the range of numeric character references is extended from 255 to
2147483645; thus, И is a valid reference to a "CYRILLIC CAPITAL
LETTER I". [ERCS] is a good source of information on Unicode and
SGML, although its scope and technical content differ greatly from
this specification.
NOTE -- the above SGML declaration, like that of HTML 2.0,
specifies the character numbers 128 to 159 (80 to 9F hex) as
UNUSED. This means that numeric character references within that
range (e.g. ’) are illegal in HTML. Neither ISO 8859-1 nor
ISO 10646 contain characters in that range, which is reserved for
control characters.
Another change was made from the HTML 2.0 SGML declaration, in the
belief that the latter did not express its authors' true intent. The
syntax character set declaration was changed from ISO 646.IRV:1983 to
the newer ISO 646.IRV:1991, the latter, but not the former, being
identical with US-ASCII. In principle, this introduces an
incompatibility with HTML 2.0, but in practice it should increase
interoperability by i) having the SGML declaration say what everyone
thinks and ii) making the syntax character set a proper subset of the
document character set. The characters that differ between the two
versions of ISO 646.IRV are not actually used to express HTML syntax.
ISO 10646-1:1993 is the most encompassing character set currently
existing, and there is no other character set that could take its
place as the document character set for HTML. If nevertheless for a
specific application there is a need to use characters outside this
standard, this should be done by avoiding any conflicts with present
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or future versions of ISO 10646, i.e. by assigning these characters
to a private zone of the UCS-4 coding space [ISO-10646 section 11].
Also, it should be borne in mind that such a use will be highly
unportable; in many cases, it may be better to use inline bitmaps.
2.3. Undisplayable characters
With the document character set being the full ISO 10646, the
possibility that a character cannot be displayed due to lack of
appropriate resources (fonts) cannot be avoided. Because there are
many different things that can be done in such a case, this document
does not prescribe any specific behaviour. Depending on the
implementation, this may also be handled by the underlaying display
system and not the application itself. The following considerations,
however, may be of help:
- A clearly visible, but unobtrusive behaviour should be preferred.
Some documents may contain many characters that cannot be
rendered, and so showing an alert for each of them is not the
right thing to do.
- In case a numeric representation of the missing character is
given, its hexadecimal (not decimal) form is to be preferred,
because this form is used in character set standards [ERCS].
3. The LANG attribute
Language tags can be used to control rendering of a marked up
document in various ways: glyph disambiguation, in cases where the
character encoding is not sufficient to resolve to a specific glyph;
quotation marks; hyphenation; ligatures; spacing; voice synthesis;
etc. Independently of rendering issues, language markup is useful as
content markup for purposes such as classification and searching.
Since any text can logically be assigned a language, almost all HTML
elements admit the LANG attribute. The DTD reflects this; the only
elements in this version of HTML without the LANG attribute are BR,
HR, BASE, NEXTID, and META. It is also intended that any new element
introduced in later versions of HTML will admit the LANG attribute,
unless there is a good reason not to do so.
The language attribute, LANG, takes as its value a language tag that
identifies a natural language spoken, written, or otherwise conveyed
by human beings for communication of information to other human
beings. Computer languages are explicitly excluded.
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The syntax and registry of HTML language tags is the same as that
defined by RFC 1766 [RFC1766]. In summary, a language tag is composed
of one or more parts: A primary language tag and a possibly empty
series of subtags:
language-tag = primary-tag *( "-" subtag )
primary-tag = 1*8ALPHA
subtag = 1*8ALPHA
Whitespace is not allowed within the tag and all tags are case-
insensitive. The namespace of language tags is administered by the
IANA. Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin
In the context of HTML, a language tag is not to be interpreted as a
single token, as per RFC 1766, but as a hierarchy. For example, a
user agent that adjusts rendering according to language should
consider that it has a match when a language tag in a style sheet
entry matches the initial portion of the language tag of an element.
An exact match should be preferred. This interpretation allows an
element marked up as, for instance, "en-US" to trigger styles
corresponding to, in order of preference, US-English ("en-US") or
'plain' or 'international' English ("en").
NOTE -- using the language tag as a hierarchy does not imply that
all languages with a common prefix will be understood by those
fluent in one or more of those languages; it simply allows the
user to request this commonality when it is true for that user.
The rendering of elements may be affected by the LANG attribute. For
any element, the value of the LANG attribute overrides the value
specified by the LANG attribute of any enclosing element and the
value (if any) of the HTTP Content-Language header. If none of these
are set, a suitable default, perhaps controlled by user preferences,
by automatic context analysis or by the user's locale, should be used
to control rendering.
4. Additional entities, attributes and elements
4.1. Full Latin-1 entity set
According to the suggestion of section 14 of [RFC1866], the set of
Latin-1 entities is extended to cover the whole right part of ISO-
8859-1 (all code positions with the high-order bit set), including
the already commonly used , © and ®. The names of the
entities are taken from the appendices of SGML [ISO-8879]. A list is
provided in section 7.3 of this specification.
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4.2. Markup for language-dependent presentation
4.2.1. Overview
For the correct presentation of text in certain languages
(irrespective of formatting issues), some support in the form of
additional entities and elements is needed.
In particular, the following features are dealt with:
- Markup of bidirectional text, i.e. text where left-to-right and
right-to-left scripts are mixed.
- Control of cursive joining behaviour in contexts where the
default behaviour is not appropriate.
- Language-dependent rendering of short (in-line) quotations.
- Better justification control for languages where this is
important.
- Superscripts and subscripts for languages where they appear as
part of general text.
Some of the above features need very little additional support;
others need more. The additional features are introduced below with
brief comments only. Explanations on cursive joining behaviour and
bidirectional text follow later. For cursive joining behaviour and
bidirectional text, this document follows [UNICODE] in that: i)
character semantics, where applicable, are identical to [UNICODE],
and ii) where functionality is moved to HTML as a higher level
protocol, this is done in a way that allows straightforward
conversion to the lower-level mechanisms defined in [UNICODE].
4.2.2. List of entities, elements, and attributes
First, a generic container is needed to carry the LANG and DIR (see
below) attributes in cases where no other element is appropriate; the
SPAN element is introduced for that purpose.
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A set of named character entities is added for use with bidirectional
rendering and cursive joining control:
These entities can be used in place of the corresponding formatting
characters whenever convenient, for example to ease keyboard entry or
when a formatting character is not available in the character
encoding of the document.
Next, an attribute called DIR is introduced, restricted to the values
LTR (left-to-right) and RTL (right-to-left), for the indication of
directionality in the context of bidirectional text (see 4.2.4 below
for details). Since any text and many other elements (e.g. tables)
can logically be assigned a directionality, all elements except BR,
HR, BASE, NEXTID, and META admit this attribute. The DTD reflects
this. It is also intended that any new element introduced in later
versions of HTML will admit the DIR attribute, unless there is a good
reason not to do so.
A new phrase-level element called BDO (BIDI Override) is introduced,
which requires the DIR attribute to specify whether the override is
left-to-right or right-to-left. This element is required for
bidirectional text control; for detailed explanations, see section
4.2.4.
The phrase-level element Q is introduced to allow language-dependent
rendering of short quotations depending on language and platform
capability. As the following examples show (rather poorly, because of
the character set restriction of Internet specifications), the
quotation marks surrounding the quotation are particularly affected:
"a quotation in English", `another, slightly better one', ,,a
quotation in German'', << a quotation in French >>. The contents of
the Q element does not include quotation marks, which have to be
added by the rendering process.
NOTE -- Q elements can be nested. Many languages use different
quotation styles for outer and inner quotations, and this should
be respected by user-agents implementing this element.
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NOTE -- minimal support for the Q element is to surround the
contents with some kind of quotes, like the plain ASCII double
quotes. As this is rather easy to implement, and as the lack of
any visible quotes may affect the perceived meaning of the text,
user-agent implementors are strongly requested to provide at least
this minimal level of support.
Many languages require superscript text for proper rendering: as an
example, the French "Mlle Dupont" should have "lle" in superscript.
The SUP element, and its sibling SUB for subscript text, are
introduced to allow proper markup of such text. SUP and SUB contents
are restricted to PCDATA to avoid nesting problems.
Finally, in many languages text justification is much more important
than it is in Western languages, and justifies markup. The ALIGN
attribute, admitting values of LEFT, RIGHT, CENTER and JUSTIFY, is
added to a selection of elements where it makes sense (the block-like
P, HR, H1 to H6, OL, UL, DIR, MENU, LI, BLOCKQUOTE and ADDRESS). If
a user-agent chooses to have LEFT as a default for blocks of left-
to-right directionality, it should use RIGHT for blocks of right-to-
left directionality.
NOTE -- RFC 1866 section 4.2.2 specifies that an HTML user agent
should treat an end of line as a word space, except in
preformatted text. This should be interpreted in the context of
the script being processed, as the way words are separated in
writing is script-dependent. For some scripts (e.g. Latin), a
word space is just a space, but in other scripts (e.g. Thai) it is
a zero-width word separator, whereas in yet other scripts (e.g.
Japanese) it is nothing at all, i.e. totally ignored.
NOTE -- the SOFT HYPHEN character (U+00AD) needs special attention
from user-agent implementers. It is present in many character
sets (including the whole ISO 8859 series and, of course, ISO
10646), and can always be included by means of the reference
. Its semantics are different from the plain HYPHEN: it
indicates a point in a word where a line break is allowed. If the
line is indeed broken there, a hyphen must be displayed at the end
of the first line. If not, the character is not dispalyed at all.
In operations like searching and sorting, it must always be
ignored.
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In the DTD, the LANG and DIR attributes are grouped together in a
parameter entity called attrs. To parallel RFC 1942 [RFC1942], the
ID and CLASS attributes are also included in attrs. The ID and CLASS
attributes are required for use with style sheets, and RFC 1942
defines them as follows:
ID Used to define a document-wide identifier. This can be used
for naming positions within documents as the destination of a
hypertext link. It may also be used by style sheets for
rendering an element in a unique style. An ID attribute value is
an SGML NAME token. NAME tokens are formed by an initial
letter followed by letters, digits, "-" and "." characters. The
letters are restricted to A-Z and a-z.
CLASS A space separated list of SGML NAME tokens. CLASS names
specify that the element belongs to the corresponding named
classes. It allows authors to distinguish different roles
played by the same tag. The classes may be used by style
sheets to provide different renderings as appropriate to
these roles.
4.2.3. Cursive joining behaviour
Markup is needed in some cases to force cursive joining behavior in
contexts in which it would not normally occur, or to block it when it
would normally occur.
The zero-width joiner and non-joiner ( and ) are used to
control cursive joining behaviour. For example, ARABIC LETTER HEH is
used in isolation to abbreviate "Hijri" (the Islamic calendrical
system); however, the initial form of the letter is desired, because
the isolated form of HEH looks like the digit five as employed in
Arabic script. This is obtained by following the HEH with a zero-
width joiner whose only effect is to provide context. In Persian
texts, there are cases where a letter that normally would join a
subsequent letter in a cursive connection does not. Here a zero-
width non- joiner is used.
4.2.4. Bidirectional text
Many languages are written in horizontal lines from left to right,
while others are written from right to left. When both writing
directions are present, one talks of bidirectional text (BIDI for
short). BIDI text requires markup in special circumstances where
ambiguities as to the directionality of some characters have to be
resolved. This markup affects the ability to render BIDI text in a
semantically legible fashion. That is, without this special BIDI
markup, cases arise which would prevent *any* rendering whatsoever
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that reflected the basic meaning of the text. Plain text may contain
BIDI markup in the form of special-purpose formatting characters.
This is also possible in HTML, which includes the five BIDI-related
formatting characters (202A - 202E) of ISO 10646. As an alternative,
HTML provides equivalent SGML markup.
BIDI is a complex issue, and conversion of logical text sequences to
display sequences has to be done according to the algorithm and
character properties specified in [UNICODE]. Here, explanations are
given only as far as they are needed to understand the necessity of
the features introduced and to define their exact semantics.
The Unicode BIDI algorithm is based on the individual characters of a
text being stored in logical order, that is the order in which they
are normally input and in which the corresponding sounds are normally
spoken. To make rendering of logical order text possible, the
algorithm assigns a directionality property to each character, e.g.
Latin letters are specified to have a left-to-right direction, Arabic
and Hebrew characters have a right-to-left direction.
The left-to-right and right-to-left marks ( and ) are used
to disambiguate directionality of neutral characters. For example,
when a double quote sits between an Arabic and a Latin letter, its
direction is ambiguous; if a directional mark is added on one side
such that the quotation mark is surrounded by characters of only one
directionality, the ambiguity is removed. These characters are like
zero width spaces which have a directional property (but no word/line
break property).
Nested embeddings of contra-directional text runs, due to nested
quotations or to the pasting of text from one BIDI context to
another, is also a case where the implicit directionality of
characters is not sufficient, requiring markup. Also, it is
frequently desirable to specify the basic directionality of a block
of text. For these purposes, the DIR attribute is used.
On block-type elements, the DIR attribute indicates the base
directionality of the text in the block; if omitted it is inherited
from the parent element. The default directionality of the overall
HTML document is left-to-right.
On inline elements, it makes the element start a new embedding level
(to be explained below); if omitted the inline element does not start
a new embedding level.
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NOTE -- the PRE, XMP and LISTING elements admit the DIR attribute.
Their contents should not be considered as preformatted with
respect to bidirectional layout, but the BIDI algorithm should be
applied to each line of text.
Following is an example of a case where embedding is needed, showing
its effect:
Given the following latin (upper case) and arabic (lower case)
letters in backing store with the specified embeddings:
AB xy CD zw
EF
One gets the following rendering (with [] showing the directional
transitions):
[ AB [ wz [ CD ] yx ] EF ]
On the other hand, without this markup and with a base direction
of LTR one gets the following rendering:
[ AB [ yx ] CD [ wz ] EF ]
Notice that yx is on the left and wz on the right unlike the above
case where the embedding levels are used. Without the embedding
markup one has at most two levels: a base directional level and a
single counterflow directional level.
The DIR attribute on inline elements is equivalent to the formatting
characters LEFT-TO-RIGHT EMBEDDING (202A) and RIGHT-TO-LEFT
EMBEDDING (202B) of ISO 10646. The end tag of the element is
equivalent to the POP DIRECTIONAL FORMATTING (202C) character.
Directional override, as provided by the BDO element, is needed to
deal with unusual short pieces of text in which directionality cannot
be resolved from context in an unambiguous fashion. For example, it
can be used to force left-to-right (or right-to-left) display of part
numbers composed of Latin letters, digits and Hebrew letters.
The effect of BDO is to force the directionality of all characters
within it to the value of DIR, irrespective of their intrinsic
directional properties. It is equivalent to using the LEFT-TO-RIGHT
OVERRIDE (202D) or RIGHT-TO-LEFT OVERRIDE (202E) characters of ISO
10646, the end tag again being equivalent to the POP DIRECTIONAL
FORMATTING (202C) character.
Yergeau, et. al. Standards Track [Page 15]
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NOTE -- authors and authoring software writers should be aware
that conflicts can arise if the DIR attribute is used on inline
elements (including BDO) concurrently with the use of the
corresponding ISO 10646 formatting characters.
Preferably one or the other should be used exclusively; the markup
method is better able to guarantee document structural integrity,
and alleviates some problems when editing bidirectional HTML text
with a simple text editor, but some software may be more apt at
using the 10646 characters. If both methods are used, great care
should be exercised to insure proper nesting of markup and
directional embedding or override; otherwise, rendering results
are undefined.
5. Forms
5.1. DTD additions
It is natural to expect input in any language in forms, as they
provide one of the only ways of obtaining user input. While this is
primarily a UI issue, there are some things that should be specified
at the HTML level to guide behavior and promote interoperability.
To ensure full interoperability, it is necessary for the user agent
(and the user) to have an indication of the character encoding(s)
that the server providing a form will be able to handle upon
submission of the filled-in form. Such an indication is provided by
the ACCEPT-CHARSET attribute of the INPUT and TEXTAREA elements,
modeled on the HTTP Accept-Charset header (see [HTTP-1.1]), which
contains a space and/or comma delimited list of character sets
acceptable to the server. A user agent may want to somehow advise
the user of the contents of this attribute, or to restrict his
possibility to enter characters outside the repertoires of the listed
character sets.
NOTE -- The list of character sets is to be interpreted as an
EXCLUSIVE-OR list; the server announces that it is ready to accept
any ONE of these character encoding schemes for each part of a
multipart entity. The client may perform character encoding
translation to satisfy the server if necessary.
NOTE -- The default value for the ACCEPT-CHARSET attribute of an
INPUT or TEXTAREA element is the reserved value "UNKNOWN". A user
agent may interpret that value as the character encoding scheme
that was used to transmit the document containing that element.
Yergeau, et. al. Standards Track [Page 16]
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5.2. Form submission
The HTML 2.0 form submission mechanism, based on the "application/x-
www-form-urlencoded" media type, is ill-equipped with regard to
internationalization. In fact, since URLs are restricted to ASCII
characters, the mechanism is akward even for ISO-8859-1 text.
Section 2.2 of [RFC1738] specifies that octets may be encoded using
the "%HH" notation, but text submitted from a form is composed of
characters, not octets. Lacking a specification of a character
encoding scheme, the "%HH" notation has no well-defined meaning.
The best solution is to use the "multipart/form-data" media type
described in [RFC1867] with the POST method of form submission. This
mechanism encapsulates the value part of each name-value pair in a
body-part of a multipart MIME body that is sent as the HTTP entity;
each body part can be labeled with an appropriate Content-Type,
including if necessary a charset parameter that specifies the
character encoding scheme. The changes to the DTD necessary to
support this method of form submission have been incorporated in the
DTD included in this specification.
A less satisfactory solution is to add a MIME charset parameter to
the "application/x-www-form-urlencoded" media type specifier sent
along with a POST method form submission, with the understanding that
the URL encoding of [RFC1738] is applied on top of the specified
character encoding, as a kind of implicit Content-Transfer-Encoding.
One problem with both solutions above is that current browsers do not
generally allow for bookmarks to specify the POST method; this should
be improved. Conversely, the GET method could be used with the form
data transmitted in the body instead of in the URL. Nothing in the
protocol seems to prevent it, but no implementations appear to exist
at present.
How the user agent determines the encoding of the text entered by the
user is outside the scope of this specification.
NOTE -- Designers of forms and their handling scripts should be
aware of an important caveat: when the default value of a field
(the VALUE attribute) is returned upon form submission (i.e. the
user did not modify this value), it cannot be guaranteed to be
transmitted as a sequence of octets identical to that in the
source document -- only as a possibly different but valid encoding
of the same sequence of text elements. This may be true even if
the encoding of the document containing the form and that used for
submission are the same.
Yergeau, et. al. Standards Track [Page 17]
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Differences can occur when a sequence of characters can be
represented by various sequences of octets, and also when a
composite sequence (a base character plus one or more combining
diacritics) can be represented by either a different but
equivalent composite sequence or by a fully precomposed character.
For instance, the UCS-2 sequence 00EA+0323 (LATIN SMALL LETTER E
WITH CIRCUMFLEX ACCENT + COMBINING DOT BELOW) may be transformed
into 1EC7 (LATIN SMALL LETTER E WITH CIRCUMFLEX ACCENT AND DOT
BELOW), into 0065+0302+0323 (LATIN SMALL LETTER E + COMBINING
CIRCUMFLEX ACCENT + COMBINING DOT BELOW), as well as into other
equivalent composite sequences.
6. External character encoding issues
Proper interpretation of a text document requires that the character
encoding scheme be known. Current HTTP servers, however, do not
generally include an appropriate charset parameter with the Content-
Type header. This is bad behaviour, which is even encouraged by the
continued existence of browsers that declare an unrecognized media
type when they receive a charset parameter. User agent
implementators are strongly encouraged to make their software
tolerant of this parameter, even if they cannot take advantage of it.
Proper labelling is highly desirable, but some preventive measures
can be taken to minimize the detrimental effects of its absence:
In the case where a document is accessed from a hyperlink in an
origin HTML document, a CHARSET attribute is added to the attribute
list of elements with link semantics (A and LINK), specifically by
adding it to the linkExtraAttributes entity. The value of that
attribute is to be considered a hint to the User Agent as to the
character encoding scheme used by the resource pointed to by the
hyperlink; it should be the appropriate value of the MIME charset
parameter for that resource.
In any document, it is possible to include an indication of the
encoding scheme like the following, as early as possible within the
HEAD of the document:
This is not foolproof, but will work if the encoding scheme is such
that ASCII-valued octets stand for ASCII characters only at least
until the META element is parsed. Note that there are better ways
for a server to obtain character encoding information, instead of the
unreliable META above; see [NICOL2] for some details and a proposal.
Yergeau, et. al. Standards Track [Page 18]
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For definiteness, the "charset" parameter received from the source of
the document should be considered the most authoritative, followed in
order of preference by the contents of a META element such as the
above, and finally the CHARSET parameter of the anchor that was
followed (if any).
When HTML text is transmitted directly in UCS-2 or UCS-4 form, the
question of byte order arises: does the high-order byte of each
multi-byte character come first or last? For definiteness, this
specification recommends that UCS-2 and UCS-4 be transmitted in big-
endian byte order (high order byte first), which corresponds to the
established network byte order for two- and four-byte quantities, to
the ISO 10646 requirement and Unicode recommendation for serialized
text data and to RFC 1641. Furthermore, to maximize chances of
proper interpretation, it is recommended that documents transmitted
as UCS-2 or UCS-4 always begin with a ZERO-WIDTH NON-BREAKING SPACE
character (hexadecimal FEFF or 0000FEFF) which, when byte-reversed
becomes number FFFE or FFFE0000, a character guaranteed to be never
assigned. Thus, a user-agent receiving an FFFE as the first octets
of a text would know that bytes have to be reversed for the remainder
of the text.
There exist so-called UCS Transformation Formats than can be used to
transmit UCS data, in addition to UCS-2 and UCS-4. UTF-7 [RFC1642]
and UTF-8 [UTF-8] have favorable properties (no byte-ordering
problem, different flavours of ASCII compatibility) that make them
worthy of consideration, especially for transmission of multilingual
text. Another encoding scheme, MNEM [RFC1345], also has interesting
properties and the capability to transmit the full UCS. The UTF-1
transformation format of ISO 10646:1993 (registered by IANA as ISO-
10646-UTF-1), has been removed from ISO 10646 by amendment 4, and
should not be used.
Yergeau, et. al. Standards Track [Page 19]
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7. HTML Public Text
7.1. HTML DTD
This section contains a DTD for HTML based on the HTML 2.0 DTD of RFC
1866, incorporating the changes for file upload as specified in RFC
1867, and the changes deriving from this document.
...
--
>
]]>
Yergeau, et. al. Standards Track [Page 20]
RFC 2070 HTML Internationalization January 1997
2045)
-->
2068
-->
1766 language tag --
DIR (ltr|rtl) #IMPLIED -- text directionnality --
ID ID #IMPLIED -- element identifier
(from RFC1942) --
CLASS NAMES #IMPLIED -- for subclassing elements
(from RFC1942) --">
%ISOlat1;
]]>
Yergeau, et. al. Standards Track [Page 24]
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Heading
is preferred to
Heading
-->
]]>
"
>
Yergeau, et. al. Standards Track [Page 25]
RFC 2070 HTML Internationalization January 1997
#AttVal(Alt)"
>
]]>
]]>
Yergeau, et. al. Standards Track [Page 27]
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]]>
Yergeau, et. al. Standards Track [Page 29]
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Directory"
>
Menu"
>
Heading
Text ...
is preferred to
Heading
Text ...
-->
]]>
Form:"
%SDASUFF; "Form End."
>
Select #AttVal(Multiple)"
>
]]>
]]>
" >
[Document is indexed/searchable.]">
Yergeau, et. al. Standards Track [Page 34]
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]]>
7.2. SGML Declaration for HTML
7.3. ISO Latin 1 entity set
The following public text lists each of the characters specified in
the Added Latin 1 entity set, along with its name, syntax for use,
and description. This list is derived from ISO Standard
8879:1986//ENTITIES Added Latin 1//EN. HTML includes the entire
entity set, and adds entities for all missing characters in the right
part of ISO-8859-1.
Yergeau, et. al. Standards Track [Page 37]
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Yergeau, et. al. Standards Track [Page 38]
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Yergeau, et. al. Standards Track [Page 39]
RFC 2070 HTML Internationalization January 1997
8. Security Considerations
Anchors, embedded images, and all other elements which contain URIs
as parameters may cause the URI to be dereferenced in response to
user input. In this case, the security considerations of [RFC1738]
apply.
The widely deployed methods for submitting form requests -- HTTP and
SMTP -- provide little assurance of confidentiality. Information
providers who request sensitive information via forms -- especially
by way of the `PASSWORD' type input field (see section 8.1.2 in
[RFC1866]) -- should be aware and make their users aware of the lack
of confidentiality.
Bibliography
[BRYAN88] M. Bryan, "SGML -- An Author's Guide to the Standard
Generalized Markup Language", Addison-Wesley, Reading,
1988.
[ERCS] Extended Reference Concrete Syntax for SGML.
[GOLD90] C. F. Goldfarb, "The SGML Handbook", Y. Rubinsky, Ed.,
Oxford University Press, 1990.
[HTTP-1.1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2068, January 1997.
[ISO-639] ISO 639:1988. International standard -- Code for the
representation of the names of languages. Technical
content in
[ISO-8859] ISO 8859. International standard -- Information pro-
cessing -- 8-bit single-byte coded graphic character
sets -- Part 1: Latin alphabet No. 1 (1987) -- Part 2:
Latin alphabet No. 2 (1987) -- Part 3: Latin alphabet
No. 3 (1988) -- Part 4: Latin alphabet No. 4 (1988) --
Part 5: Latin/Cyrillic alphabet (1988) -- Part 6:
Latin/Arabic alphabet (1987) -- Part : Latin/Greek
alphabet (1987) -- Part 8: Latin/Hebrew alphabet
(1988) -- Part 9: Latin alphabet No. 5 (1989) -- Part
10: Latin alphabet No. 6 (1992)
Yergeau, et. al. Standards Track [Page 40]
RFC 2070 HTML Internationalization January 1997
[ISO-8879] ISO 8879:1986. International standard -- Information
processing -- Text and office systems -- Standard gen-
eralized markup language (SGML).
[ISO-10646] ISO/IEC 10646-1:1993. International standard -- Infor-
mation technology -- Universal multiple-octet coded
character Sset (UCS) -- Part 1: Architecture and basic
multilingual plane.
[NICOL] G.T. Nicol, "The Multilingual World Wide Web",
Electronic Book Technologies, 1995,
[NICOL2] G.T. Nicol, "MIME Header Supplemented File Type", Work
in Progress, EBT, October 1995.
[RFC1345] Simonsen, K., "Character Mnemonics & Character Sets",
RFC 1345, Rationel Almen Planlaegning, June 1992.
[RFC1468] Murai, J., Crispin M., and E. van der Poel,
"Japanese Character Encoding for Internet Messages",
RFC 1468, Keio University, Panda Programming, June
1993.
[RFC2045] Freed, N., and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, Innosoft, First Virtual,
November 1996.
[RFC1641] Goldsmith, D., and M.Davis, "Using Unicode with MIME",
RFC 1641, Taligent inc., July 1994.
[RFC1642] Goldsmith, D., and M. Davis, "UTF-7: A Mail-safe
Transformation Format of Unicode", RFC 1642, Taligent,
Inc., July 1994.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill,
"Uniform Resource Locators (URL)", RFC 1738, CERN,
Xerox PARC, University of Minnesota, October 1994.
[RFC1766] Alverstrand, H., "Tags for the Identification of
Languages", RFC 1766, UNINETT, March 1995.
[RFC1866] Berners-Lee, T., and D. Connolly, "Hypertext Markup
Language - 2.0", RFC 1866, MIT/W3C, November 1995.
[RFC1867] Nebel, E., and L. Masinter, "Form-based File Upload
in HTML", RFC 1867, Xerox Corporation, November 1995.
Yergeau, et. al. Standards Track [Page 41]
RFC 2070 HTML Internationalization January 1997
[RFC1942] Raggett, D., "HTML Tables", RFC 1942, W3C, May 1996.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2068, January 1997.
[SQ91] SoftQuad, "The SGML Primer", 3rd ed., SoftQuad Inc.,
1991.
[TAKADA] Toshihiro Takada, "Multilingual Information Exchange
through the World-Wide Web", Computer Networks and
ISDN Systems, Vol. 27, No. 2, Nov. 1994 , p. 235-241.
[TEI] TEI Guidelines for Electronic Text Encoding and Inter-
change.
[UNICODE] The Unicode Consortium, "The Unicode Standard --
Worldwide Character Encoding -- Version 1.0", Addison-
Wesley, Volume 1, 1991, Volume 2, 1992, and Technical
Report #4, 1993. The BIDI algorithm is in appendix A
of volume 1, with corrections in appendix D of volume
2.
[UTF-8] ISO/IEC 10646-1:1993 AMENDMENT 2 (1996). UCS Transfor-
mation Format 8 (UTF-8).
[VANH90] E. van Hervijnen, "Practical SGML", Kluwer Academicq
Publishers Group, Norwell and Dordrecht, 1990.
Yergeau, et. al. Standards Track [Page 42]
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Authors' Addresses
Frangois Yergeau
Alis Technologies
100, boul. Alexis-Nihon, bureau 600
Montrial QC H4M 2P2
Canada
Tel: +1 (514) 747-2547
Fax: +1 (514) 747-2561
EMail: fyergeau@alis.com
Gavin Thomas Nicol
Electronic Book Technologies, Japan
1-29-9 Tsurumaki,
Setagaya-ku,
Tokyo
Japan
Tel: +81-3-3230-8161
Fax: +81-3-3230-8163
EMail: gtn@ebt.com, gtn@twics.co.jp
Glenn Adams
Spyglass
118 Magazine Street
Cambridge, MA 02139
U.S.A.
Tel: +1 (617) 864-5524
Fax: +1 (617) 864-4965
EMail: glenn@spyglass.com
Martin J. Duerst
Multimedia-Laboratory
Department of Computer Science
University of Zurich
Winterthurerstrasse 190
CH-8057 Zurich
Switzerland
Tel: +41 1 257 43 16
Fax: +41 1 363 00 35
EMail: mduerst@ifi.unizh.ch
Yergeau, et. al. Standards Track [Page 43]