W3C
HTML 4.0 Guidelines for Mobile Access
W3C Note - 15 March 1999
This version:
Latest version:
Editors:
Tomihisa Kamada, ACCESS Co.,Ltd.
Takuya Asada, W3C
Masayasu Ishikawa, W3C
Shin'ichi Matsui, W3C/Panasonic
Copyright (c) 1998-1999 W3C (MIT, INRIA, Keio), All Rights Reserved.
W3C liability, trademark, document use and software licensing rules
apply.
Status of This Document
This document is a Note made available by the World Wide Web
Consortium (W3C) for discussion. This document may be updated,
replaced or rendered obsolete by other documents at any time. It is
inappropriate to use this document as reference material or to cite
them as other than "work in progress". This is work in progress, and
does not imply endorsement by, or the consensus of, either W3C
membership or members of the Mobile Access Interest Group (members
only).
This document has been produced as part of the W3C Mobile Access
Activity. Please send comments about this document to the public
mailing list www-mobile@w3.org (archive).
Abstract
This document describes guidelines for content authors how to create
HTML 4.0 contents to be acceptable to mobile devices as much as
possible.
Available Formats
This document is available in the following formats:
HTML:
A plain text file:
HTML as a gzip'ed tar file, including accompanying resources:
HTML as a zip file, including accompanying resources:
In case of a discrepancy between the various formats of the document,
is considered
the definitive version.
Table of Contents
1. Introduction
2. Requirements from Mobile Access
1. Hardware Restrictions
2. Narrow Bandwidth Networking
3. Accessibility Requirements
3. Guidelines for Mobile Access
1. Styles
2. Scripts, Event Handlers
3. Forms
4. Frames
5. Image Maps
6. Tables
7. Objects and Images
8. Access keys
4. A Sample HTML 4.0 DTD for Mobile Access
5. Concluding Remarks and Future Directions
* References
* Appendix HTML 4.0 Mobile DTD
+ Sample SGML catalog
+ Document Type Definition
1. Introduction
Ever since the Internet industry exploded a few years ago, numerous
related industries have spawned and flourished from the standard WWW
(World Wide Web) infrastructure. This growth is most attributable to
the exponential growth of Internet users all over the world. Today,
Internet access is no longer limited to personal computers and
powerful workstations in your office, but is reaching into the your
home, as well as on the road.
A new class of electronics devices with Internet access capability
called "Information Appliances" was recently born. This Internet
access capability is embedded in devices such as televisions, set top
boxes, home game machines, telephone-based terminals, PDAs, car
navigation systems and cellular phones. These Internet appliances will
drive the merger of wireless and wired Internet world that will
eventually create a much larger industry than today's predominantly
wired Internet industry.
The need to support this new class of information appliances while
still meeting the W3C standards and the rigid design requirements are
the challenges this evolving industry faces today.
This document describes guidelines for content authors how to use HTML
4.0 Specification [HTML40] effectively for "Mobile Access", and some
design notes for user agent manufacturers.
2. Requirements from Mobile Access
There are a wide range of mobile devices in the market. The typical
product examples are as follows:
* PDA
* mobile phone
* pager
* embedded system
* automobile information appliance (car navigation system)
* mobile game machine
* digital book reader
* smart watch
This section summarizes the requirements from "Mobile Access".
2.1. Hardware Restrictions
As described in [CompactHTML], there are several hardware restrictions
in mobile devices. The major characteristics are as follows:
* small memory capacity
* small display space
* less powerful CPU
* limited input method
* no or small data storage
* limited power consumption
These characteristics have to be taken into account in designing the
user agent functionality for mobile devices, as well as creating
mobile-friendly contents.
2.2. Narrow Bandwidth Networking
The size of contents should be small in the case of narrow bandwidth
and low speed wireless networking. The major characteristics of
today's wireless connection are as follows:
* slow speed
* unstable
* expensive
* non-IP connection
These restrictions depend on underlying communication bearers. The
infrastructure will be improved in the third generation wireless phone
networking.
2.3. Accessibility Requirements
Simple and easy operations of information appliances are very
important. Unlike PC user interface, keyboard and mouse are not
suitable for mobile devices. Rather, intuitive operation like a
four-button interface (cursor forward, cursor back, select, and back
buttons) is required. In this sense, some kind of accessibility
considerations, as described in WAI Accessibility Guidelines, should
be taken into account. In general, WAI Accessiblity Guidelines are
also useful for mobile access.
For example, the "accesskey" attribute of HTML 4.0 can be used to
assign buttons to links or form selections. Also, alternative text for
images should be provided via "alt" attribute so that mobile devices
can make use of text descriptions instead of large images. For more
information, see section 3.8 and 3.7 respectively.
3. Guidelines for Mobile Access
This section describes the detail of the HTML 4.0 guidelines for
mobile access.
In order to meet the requirements as described in section 2, we use
HTML 4.0 Strict, which excludes the presentation attributes and
elements, as a suitable baseline for mobile access. Thus, the
following guidelines assume to use HTML 4.0 Strict.
3.1. Styles
Usual mobile devices have small size of screens, and limited display
capabilities, thus in many cases style sheets are not supported, or
its support is limited.
When style sheets are used, external style sheets are recommended from
the viewpoint of content size and separation of structure and style.
In this way, user agents which don't support style sheets don't have
to load unnecessary style sheets.
The "media" attribute can be used so that user agents can select
appropriate style sheets. See section 14.2.4 of [HTML40] for more
detail.
The STYLE element can be also used, but inline style sheets via
"style" attribute is not preferred, because it's not flexible and
almost like the presentation attributes which are deprecated and
excluded from the HTML 4.0 Strict.
Contents should be readable without style sheets so that devices which
don't support style sheets can still render contents reasonably.
See also section 2.10 of Techniques for [WAI-WEBCONTENT].
Note for User Agents:
It is desirable that even if user agents don't support style sheets,
it should parse STYLE elements not to show styles nor to cause any
other side-effects.
3.2. Scripts, Event Handlers
Usual mobile devices have limitation of memory storage, CPU power and
so on, thus in many cases scripting are not supported. It should not
be assumed that scripts will always be executed. Contents should be
readable even if scripts are not executed.
See also section 2.13 of Techniques for [WAI-WEBCONTENT].
Note for User Agents:
It is desirable that even if user agents don't support scripts, it
should parse SCRIPT elements not to show scripts nor to cause any
other side-effects.
3.3. Forms
Usual mobile devices support basic forms, but they don't have
keyboards like desktop PCs. Content authors should keep in mind that
it will be hard for users of mobile devices to input many characters.
Since sometimes mobile devices don't have local file systems, some
features, which depend on local file system, such as file upload,
should not be used.
Many mobile devices do not support images, thus value "image" for
"type" attribute of INPUT element should not be used.
As described in 3.2, authors should not rely on scripting, and should
avoid using attributes for event handling, such as onsubmit.
See also section 2.12 of Techniques for [WAI-WEBCONTENT].
3.4. Frames
Usual mobile devices have small size of screens, and some of them
support only textual information. Frames strongly depend on screen
interface, and are not part of the HTML 4.0 Strict. Therefore, frames
should not be used.
If there is compelling reason to use frames, make sure that those
contents have the descriptions below:
* Provide a fallback content for contents that contain frames using
NOFRAMES at the end of each FRAMESET.
* Name each frame via the "title" attribute on FRAME elements so
that users can keep track of frames by name.
See also section 2.11 of Techniques for [WAI-WEBCONTENT].
Note for User Agents:
If user agents don't support frames, it should render contents of the
NOFRAMES element. Also, even if user agents can't render frames, it is
desirable to provide links to each frame, so that users can access to
contents of each frame.
3.5. Image Maps
On mobile devices, it cannot be generally assumed that images are
always rendered, nor can be pointed by pointing devices like mouse.
Therefore, server-side image maps should not be used.
On the other hand, client-side image maps can be used even if images
are not rendered nor can be pointed, so authors may include
client-side image maps into their contents. But this is acceptable
only when alternative descriptions are provided via "alt" attribute of
AREA element, which is a required attribute in HTML 4.0, as in many
cases images are not rendered on mobile devices, or pointing on images
is impossible even if images can be rendered, as described above.
See also section 2.7 of Techniques for [WAI-WEBCONTENT].
Note for User Agents:
It will be desirable that user agents render images and provide
selections with pointing (using area information) if possible. Even if
rendering images or selection with pointing are impossible, they
should provide access to linked resource of each area via "alt"
attribute of AREA element.
3.6. Tables
Many mobile devices have small size of screens. If contents have table
descriptions, it will be very difficult to browse them in mobile
devices. In addition, though it would be possible to restructure
tables in accessible way by following guidelines as described in
Guideline 7 of [WAI-WEBCONTENT], it will be difficult for mobile
devices to process complex tables due to the hardware restrictions as
described in 2.1. It would be, therefore, safe to avoid using tables
whenever possible. Consider alternative structures, e.g. lists,
whenever appropriate. Content authors should not expect tables will
always be processed in mobile devices.
Note for User Agents:
Even if it would be difficult, user agents may parse and render tables
if possible. For example, the CAPTION element and "summary" attribute
are usable to summarize tables. See also [WAI-USERAGENT].
3.7. Objects and Images
The OBJECT elements, which are designed to embed various objects
including images into HTML documents, are also useful in mobile
devices. For example, contents which have voice (speech) contents for
cell phones and contents which have graphical maps for car navigation
systems may become typical contents for those devices, as these
contents match essential needs and functions of them.
When embedding objects via OBJECT element, alternative contents should
be provided by the fallback mechanism of OBJECT element. These
alternative contents should end up with texts.
An example below is quoted from [HTML40]:
In general, images should be embedded via OBJECT element, as it has
better fallback mechanism, but IMG element may also be used. In this
case, alternative description via "alt" attribute, which is required
in [HTML40], should be provided for user agents which cannot or do not
render images. This guideline doesn't specify nor recommend specific
formats for images.
If the image represents an important information, a long description
should be provided via longdesc attribute of IMG element, in addition
to alternative text via alt attribute.
The APPLET element, which is not part of the HTML 4.0 Strict, should
not be used in favor of the OBJECT element.
Note for User Agents:
User agents should try to render nested objects according to the
precedence rules described in section 13.3.1 of [HTML40]. Even if user
agents don't support OBJECT elements, they should render alternative
content of OBJECT element as a fallback.
If a long description is provided via longdesc attribute of IMG
element, user agents should provide access to that long description.
3.8. Access keys
In general, it cannot be assumed that input methods which consists of
a (full) keyboard and a pointing device such as mouse are available in
mobile devices. It is desirable that description for efficient use of
input methods on mobile devices are provided in contents.
From this viewpoint, assigning access keys via "accesskey" attribute
(for A, AREA, BUTTON, INPUT, LABEL, LEGEND and TEXTAREA elements) will
be effective when it is available. This may improve accessibility of
links or form related operations.
But the use of "accesskey" attribute needs careful consideration.
1. In many mobile devices, available keys are limited. It cannot be
assumed that all keys in "full" keyboards are available.
2. Available keys differ among mobile devices. For example, cell
phones will usually have "0"-"9", "*" and "#" keys, but the same
assumption cannot be applied for most pagers or mobile game
machines.
3. There may be some mobile devices that cannot use "accesskey"
attribute at all. For example, devices only for voice browsing
will not use access "keys". Content authors should not rely on
access keys for navigation.
Note for User Agents:
User agents should parse and activate access keys specified via
"accesskey" attribute whenever possible, but they should ignore access
keys when those keys are not available.
4. A Sample HTML 4.0 DTD for Mobile Access
As mentioned in section 3, HTML 4.0 Strict will be suitable as a
baseline for mobile access. This section explains a sample description
of DTD, called HTML 4.0 Mobile, as a subset of the HTML 4.0 Strict DTD
in order to reflect the guidelines described in section 3.
This DTD is also designed to be a subset of ISO/IEC 15445, a.k.a.
ISO-HTML [ISO-HTML]. ISO-HTML is a well-defined subset of HTML 4.0
Strict, so conforming to this DTD will ensure interoperability across
wide variety of platforms, including mobile devices. The complete
definition of DTD for HTML 4.0 Mobile is described in the Appendix.
Note: At the time of writing this document, [ISO-HTML] is still under
work in progress. It has not yet established as an International
Standard.
The HTML 4.0 Mobile DTD is provided to help authors creating documents
following the guidelines described in this document (e.g. by
validating with an SGML parser like nsgmls [SP]). Not all guidelines
can be expressed in the DTD, but hopefully, this DTD will make it
easier to follow these guidelines.
Typically, this DTD can be invoked as follows:
...
For example, this DTD excludes style attribute and event handlers like
onclick, so when content authors validate their documents against this
DTD, those attributes will be flagged as unknown attributes and then
authors can be aware that those attributes are not recommended. If
documents are validated against this DTD, those documents can be
labeled as valid HTML 4.0 Strict.
Note. If you use the following DOCTYPE internal subset:
]>
...
then you can enforce the strict heading order as required in
[ISO-HTML].
In any case, ensuring the validity of documents is quite important,
particularly for mobile access, because most mobile devices cannot
afford to perform costly error recovery.
5. Concluding Remarks and Future Directions
We believe that wireless network services should not be isolated from
the growing Internet services. This guidelines can be used for this
purpose. It can be referred as recommended guidelines for HTML mobile
Internet services and HTML browsing software for a variety of mobile
devices.
Currently the work for the next generation of HTML [XHTML] is going
on, which will be a family of modularized document types that will
make it easier to extend and subset HTML. Since mobile device category
is one of major targets for [XHTML], mobile profile(s) and mobile
module(s) will be defined as part of [XHTML].
Note that the effort for converging [XHTML] and the Wireless Markup
Language [WML], for the Wireless Application Protocol (WAP), is
currently ongoing between the W3C and the WAP Forum [WAP-W3C, WML1.1].
Hopefully, [WML] will be among the family of [XHTML] document types in
the future.
The worldwide standard for wireless digital phone networking protocols
(IMT-2000) will be established very soon. New services based on
IMT-2000 standard will start in the early 21st century. The bandwidth
of the network is expected to be wide enough for IP connections and
Internet-oriented contents.
This implies that HTML and related recommendations for mobile Internet
will become more and more important. This HTML 4.0 guidelines for
mobile access will be a good step towards future HTML modularity for
mobile phones.
References
[CompactHTML]
"Compact HTML for Small Information Appliances", W3C Note, T.
Kamada, 9 February 1998.
Available at
[HTML40]
"HTML 4.0 Specification", W3C Recommendation, D. Raggett, A.
Le Hors, I. Jacobs, 18 December 1997, revised 24 April 1998.
Available at
"HTML 4.0 Specification Errata" is available at
[ISO-HTML]
"ISO/IEC 15445:1999 Information Technology -- Document
Description and Processing Languages -- HyperText Markup
Language (HTML)", R. Price, D. Abrahamson, work in progress.
Available at
See also: "User's Guide to ISO/IEC 15445:1999 HyperText Markup
Language (HTML)", available at
[SP]
SP is a free, object-oriented toolkit for SGML parsing and
entity management written by James Clark. Further information
is available at .
[WAI-WEBCONTENT]
"Web Content Accessibility Guidelines", W3C Working Draft, W.
Chisholm, G. Vanderheiden, I. Jacobs, work in progress.
See also: "Techniques for Web Content Accessibility Guidelines"
The latest version of these documents can be available from
[WAI-USERAGENT]
"User Agent Accessibility Guidelines", W3C Working Draft, J.
Gunderson, I. Jacobs, work in progress.
The latest version of this document can be available from
[WAP-W3C]
"WAP Forum - W3C Cooperation White Paper", W3C Note, J. Hjelm,
B. Martin, P. King, 30 October 1998.
Available at
[WML]
"Wireless Markup Language Specification", WAP Forum, 30 April
1998.
Available at
[WML1.1]
"Wireless Markup Language Specification Version 1.1", proposed,
WAP Forum, 3 February 1999.
Available at
[XHTML]
"XHTML 1.0: The Extensible HyperText Markup Language - A
Reformulation of HTML 4.0 in XML 1.0", W3C Working Draft,
Steven Pemberton et al., work in progress.
Available at
Appendix HTML 4.0 Mobile DTD
Sample SGML catalog
Note. The HTML 4.0 Mobile DTD is a complete subset of the HTML 4.0
Strict DTD, and if you use the following DTD:
%Mobile.dtd;
then you can get the same definition as the HTML 4.0 Strict DTD, as
amended by the HTML 4.0 Specification Errata.
The following sample SGML catalog uses the above DTD to invoke the
HTML 4.0 Strict DTD. Typically, the Mobile DTD itself will be invoked
by a system identifier, so it is not included in this catalog.
-- HTML 4.0 Strict DTD, as amended by the HTML 4.0 Specification Errata --
PUBLIC "-//W3C//DTD HTML 4.0//EN" html40-strict.dtd
-- entity sets for HTML 4.0 --
PUBLIC "-//W3C//ENTITIES Latin1//EN//HTML" HTMLlat1.ent
PUBLIC "-//W3C//ENTITIES Special//EN//HTML" HTMLspecial.ent
PUBLIC "-//W3C//ENTITIES Symbols//EN//HTML" HTMLsymbol.ent
SGMLDECL html40.decl
DOCTYPE html html40-strict.dtd
Document Type Definition
]]>
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%HTMLlat1;
%HTMLsymbol;
%HTMLspecial;
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