Network Working Group D. Connolly
Request for Comments: 2854 World Wide Web Consortium (W3C)
Obsoletes: 2070, 1980, 1942, 1867, 1866 L. Masinter
Category: Informational AT&T
June 2000
The 'text/Html' Media Type
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document summarizes the history of HTML development, and defines
the "text/html" MIME type by pointing to the relevant W3C
recommendations; it is intended to obsolete the previous IETF
documents defining HTML, including RFC1866, RFC1867, RFC1980, RFC
1942 and RFC2070, and to remove HTML from IETF Standards Track.
This document was prepared at the request of the W3C HTML working
group. Please send comments to www-html@w3.org, a public mailing list
with archive at <http://lists.w3.org/Archives/Public/www-html/>.
1. IntrodUCtion and background
HTML has been in use in the World Wide Web information infrastructure
since 1990, and specified in various informal documents. The
text/html media type was first officially defined by the IETF HTML
working group in 1995 in [HTML20]. Extensions to HTML were proposed
in [HTML30], [UPLOAD], [TABLES], [CLIMAPS], and [I18N].
The IETF HTML working group closed Sep 1996, and work on defining
HTML moved to the World Wide Web Consortium (W3C). The proposed
extensions were incorporated to some extent in [HTML32], and to a
larger extent in [HTML40]. The definition of multipart/form-data from
[UPLOAD] was described in [FORMDATA]. In addition, a reformulation of
HTML 4.0 in XML 1.0[XHTML1] was developed.
[HTML32] notes "This specification defines HTML version 3.2. HTML 3.2
aims to capture recommended practice as of early '96 and as such to
be used as a replacement for HTML 2.0 (RFC1866)." Subsequent
specifications for HTML describe the differences in each version.
In addition to the development of standards, a wide variety of
additional extensions, restrictions, and modifications to HTML were
popularized by NCSA's Mosaic system and subsequently by the
competitive implementations of Netscape Navigator and Microsoft
Internet EXPlorer; these extensions are documented in numerous books
and online guides.
2. Registration of MIME media type text/html
MIME media type name: text
MIME suBType name: html
Required parameters: none
Optional parameters:
charset
The optional parameter "charset" refers to the character
encoding used to represent the HTML document as a sequence of
bytes. Any registered IANA charset may be used, but UTF-8 is
preferred. Although this parameter is optional, it is strongly
recommended that it always be present. See Section 6 below for
a discussion of charset default rules.
Note that [HTML20] included an optional "level" parameter; in
practice, this parameter was never used and has been removed from
this specification. [HTML30] also suggested a "version"
parameter; in practice, this parameter also was never used and has
been removed from this specification.
Encoding considerations:
See Section 4 of this document.
Security considerations:
See Section 7 of this document.
Interoperability considerations:
HTML is designed to be interoperable across the widest possible
range of platforms and devices of varying capabilities. However,
there are contexts (platforms of limited display capability, for
example) where not all of the capabilities of the full HTML
definition are feasible. There is ongoing work to develop both a
modularization of HTML and a set of profiling capabilities to
identify and negotiate restricted (and extended) capabilities.
Due to the long and distributed development of HTML, current
practice on the Internet includes a wide variety of HTML variants.
Implementors of text/html interpreters must be prepared to be
"bug-compatible" with popular browsers in order to work with many
HTML documents available the Internet.
Typically, different versions are distinguishable by the DOCTYPE
declaration contained within them, although the DOCTYPE
declaration itself is sometimes omitted or incorrect.
Published specification:
The text/html media type is now defined by W3C Recommendations;
the latest published version is [HTML401]. In addition, [XHTML1]
defines a profile of use of XHTML which is compatible with HTML
4.01 and which may also be labeled as text/html.
Applications which use this media type:
The first and most common application of HTML is the World Wide
Web; commonly, HTML documents contain URI references [URI] to
other documents and media to be retrieved using the HTTP protocol
[HTTP]. Many gateway applications provide HTML-based interfaces to
other underlying complex services. Numerous other applications now
also use HTML as a convenient platform-independent multimedia
document representation.
Additional information:
Magic number:
There is no single initial string that is always present for
HTML files. However, Section 5 below gives some guidelines for
recognizing HTML files.
File extension:
The file extensions 'html' or 'htm' are commonly used, but
other extensions denoting file formats for preprocessing are
also common.
Macintosh File Type code: TEXT
Person & email address to contact for further information:
Dan Connolly <connolly@w3.org>
Larry Masinter <lmm@acm.org>
Intended usage: COMMON
Author/Change controller:
The HTML specification is a work product of the World Wide Web
Consortium's HTML Working Group. The W3C has change control over
the HTML specification.
Further information:
HTML has a means of including, by reference via URI, additional
resources (image, video clip, applet) within the base document. In
order to transfer a complete HTML object and the included
resources in a single MIME object, the mechanisms of [MHTML] may
be used.
3. Fragment Identifiers
The URI specification [URI] notes that the semantics of a fragment
identifier (part of a URI after a "#") is a property of the data
resulting from a retrieval action, and that the format and
interpretation of fragment identifiers is dependent on the media type
of the retrieval result.
For documents labeled as text/html, the fragment identifier
designates the correspondingly named element; any element may be
named with the "id" attribute, and A, APPLET, FRAME, IFRAME, IMG and
MAP elements may be named with a "name" attribute. This is described
in detail in [HTML40] section 12.
4. Encoding considerations
Because of the availability within HTML itself for using character
entity references, documents that use a wide repertoire of characters
may still be represented using the US-ASCII charset and transported
without encoding. However, transport of text/html using a charset
other than US-ASCII may require base64 or quoted-printable encoding
for 7-bit channels.
As with all MIME text subtypes, the canonical form of "text/html"
must always represent a line break as a sequence of a CR byte value
(0x0D) followed by an LF (0x0A) byte value. Similarly, any
occurrence of such a CRLF sequence in "text/html" must represent a
line break. Use of CR byte values and LF byte values outside of line
break sequences is also forbidden. This rule applies regardless of
the character encoding ('charset') involved.
Note, however, that the HTTP protocol allows the transport of data
not in canonical form, and, in particular, with other end-of-line
conventions; see [HTTP] section 3.7.1. This exception is commonly
used for HTML.
HTML sent via email is still subject to the MIME restrictions; this
is discussed fully in [MHTML] Section 10.
5. Recognizing HTML files
Almost all HTML files have the string "<html" or "<HTML" near the
beginning of the file.
Documents conformant to HTML 2.0, HTML 3.2 and HTML 4.0 will start
with a DOCTYPE declaration "<!DOCTYPE HTML" near the beginning,
before the "<html". These dialects are case insensitive. Files may
start with white space, comments (introduced by "<!--" ), or
processing instructions (introduced by "<?") prior to the DOCTYPE
declaration.
XHTML documents (optionally) start with an XML declaration which
begins with "<?xml" and are required to have a DOCTYPE declaration
"<!DOCTYPE html".
6. Charset default rules
The use of an explicit charset parameter is strongly recommended.
While [MIME] specifies "The default character set, which must be
assumed in the absence of a charset parameter, is US-ASCII." [HTTP]
Section 3.7.1, defines that "media subtypes of the 'text' type are
defined to have a default charset value of 'ISO-8859-1'". Section
19.3 of [HTTP] gives additional guidelines. Using an explicit
charset parameter will help avoid confusion.
Using an explicit charset parameter also takes into account that the
overwhelming majority of deployed browsers are set to use something
else than 'ISO-8859-1' as the default; the actual default is either a
corporate character encoding or character encodings widely deployed
in a certain national or regional community. For further
considerations, please also see Section 5.2 of [HTML40].
7. Security Considerations
[HTML401], section B.10, notes various security issues with
interpreting anchors and forms in HTML documents.
In addition, the introduction of scripting languages and interactive
capabilities in HTML 4.0 introduced a number of security risks
associated with the automatic execution of programs written by the
sender but interpreted by the recipient. User agents executing such
scripts or programs must be extremely careful to insure that
untrusted software is executed in a protected environment.
8. Authors' Addresses
Daniel W. Connolly
World Wide Web Consortium (W3C)
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
EMail: connolly@w3.org
http://www.w3.org/People/Connolly/
Larry Masinter
AT&T
75 Willow Road
Menlo Park, CA 94025
EMail: LM@att.com
http://larry.masinter.net
9. References
[CLIMAPS] Seidman, J., "A Proposed Extension to HTML: Client-Side
Image Maps", RFC1980, August 1996.
[FORMDATA] Masinter, L., "Returning Values from Forms:
multipart/form-data", RFC2388, August 1998.
[HTML20] Berners-Lee, T. and D. Connolly, "Hypertext Markup
Language - 2.0", RFC1866, November 1995.
[HTML30] Raggett, D., "HyperText Markup Language Specification
Version 3.0", September 1995. (Available at
<http://www.w3.org/MarkUp/html3/CoverPage>).
[HTML32] Raggett, D., "HTML 3.2 Reference Specification", W3C
Recomendation, January 1997.
Available at <http://www.w3.org/TR/REC-html32>.
[HTML40] Raggett, D., et al., "HTML 4.0 Specification", W3C
Recommendation, December 1997.
Available at <http://www.w3.org/TR/1998/REC-html40-
19980424>
[HTML401] Raggett, D., et al., "HTML 4.01 Specification", W3C
Recommendation, December 1999.
Available at <http://www.w3.org/TR/html401>.
[HTTP] Gettys, J., Fielding, R., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC2616, June 1999.
[I18N] Yergeau, F., Nicol, G. and M. Duerst,
"Internationalization of the Hypertext Markup Language",
RFC2070, January 1997.
[MHTML] Palme, J., Hotmann, A. and N. Shelness, "MIME
Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC2557, March 1999.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC2046,
November 1996.
[TABLES] Raggett, D., "HTML Tables", RFC1942, May 1996.
[UPLOAD] Nebel, E. and L. Masinter, "Form-based File Upload in
HTML", RFC1867, November 1995.
[URI] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC2396,
August 1998.
[XHTML1] "XHTML 1.0: The Extensible HyperText Markup Language: A
Reformulation of HTML 4 in XML 1.0", W3C Recommendation,
January 2000. Available at <http://www.w3.org/TR/xhtml1>.
10. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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