Internet-Draft D. Connolly
World Wide Web Consortium (W3C)
Category: Informational L. Masinter
Xerox Corporation
draft-connolly-text-html-01.txt October 13, 1999
Obsoletes: RFC 1866, RFC 2070, RFC 1980, RFC 1867, RFC 1942
The 'text/html' Media Type
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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Copyright Notice
Copyright (C) The Internet Society (1999). 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 RFC 1866, RFC 1867, RFC 1980,
RFC 1942 and RFC 2070, 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 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 is being developed [XHTML1].
[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 (RFC 1866)."
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 [HTML40]. As of this writing, a
revision, HTML 4.01 [HTML401], is being developed as a revision.
In addition, [XHTML1], also a work in progress, defines a profile
of use of XHTML which is compatible with HTML 4.0 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: HTML
Person & email address to contact for further information:
Dan Connolly <connolly@w3.org>
Larry Masinter <masinter@parc.xerox.com>
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 A element (named with a "name"
attribute), or any other element (named with the an "id"
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 for non-ASCII characters, it is possible that
text/html documents with a wide repertoire of characters may be
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.
The canonical form of any MIME "text" subtype MUST always represent
a line break as a CRLF sequence. Similarly, any occurrence of CRLF
in MIME "text" MUST represent a line break. Use of CR and LF
outside of line break sequences is also forbidden. This rule
applies regardless of format or character set or sets involved.
Note, however, that HTTP 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.
7. Security Considerations
[HTML40], 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. Author's Address
Daniel W. Connolly
World Wide Web Consortium (W3C)
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
phone:+1-512-310-2971
mailto:connolly@w3.org
http://www.w3.org/People/Connolly/
Larry Masinter
Palo Alto Research Center
Xerox Corporation
3333 Coyote Hill Road
Palo Alto, CA 94304
mailto: masinter@parc.xerox.com
http://purl.org/NET/masinter
9. References
[HTML30] "HyperText Markup Language Specification Version 3.0." Dave
Raggett, September 1995. Internet Draft (expired). Available
at <http://www.w3.org/MarkUp/html3/CoverPage>.
[HTML20] "Hypertext Markup Language - 2.0." T. Berners-Lee &
D. Connolly. RFC 1866. November 1995. Additional information
available at <http://www.w3.org/MarkUp/html-spec/>.
[UPLOAD] "Form-based File Upload in HTML." E. Nebel & L. Masinter. RFC
1867. November 1995.
[TABLES] "HTML Tables." D. Raggett. RFC 1942. May 1996.
[CLIMAPS] "A Proposed Extension to HTML : Client-Side Image Maps."
J. Seidman. RFC 1980. August 1996.
[MIME] "Multipurpose Internet Mail Extensions (MIME) Part Two: Media
Types." N. Freed & N. Borenstein. November 1996. RFC 2046.
[HTML32] "HTML 3.2 Reference Specification." Dave Raggett. W3C
Recomendation. 14 January 1997. Available at
<http://www.w3.org/TR/REC-html32>.
[I18N] "Internationalization of the Hypertext Markup Language." RFC
2070. F. Yergeau, G. Nicol, G. Adams, M. Duerst. January
1997.
[FORMDATA] "Returning Values from Forms: multipart/form-data". RFC
2388. L. Masinter. August 1998.
[HTML40] "HTML 4.0 Specification." Raggett, Le Hors, Jacobs. W3C
Recommendation. 18 Dec 1997. Available at
<http://www.w3.org/TR/REC-html40>.
[HTML401] "HTML 4.01 Specification." D. Raggett, A. Le Hors,
I. Jacobs. W3C Proposed Recommendation (work in progress),
August 1999. Available at
<http://www.w3.org/TR/1999/PR-html40-19990824>.
[XHTML1] "XHTML 1.0: The Extensible HyperText Markup Language: A
Reformulation of HTML 4.0 in XML 1.0." W3C HTML Working
Group. W3C Proposed Recommendation (work in progress). August
1999. Available at <http://www.w3.org/TR/xhtml1>.
[MHTML] "MIME Encapsulation of Aggregate Documents, such as
HTML (MHTML)". J. Palme, A. Hopmann, N. Shelness.
March 1999. RFC 2557.
[URI] "Uniform Resource Identifiers (URI): Generic Syntax."
T. Berners-Lee, R. Fielding, L. Masinter. August 1998,
RFC 2396.
[HTTP] "Hypertext Transfer Protocol -- HTTP/1.1." R. Fielding,
J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach,
T. Berners-Lee. June 1999.RFC 2616.