Abstract
The increasing disparities between the capabilities of
different kinds of Web user agents present challenges to Web
content developers wishing to reach a wide audience. A promising
approach is to formally describe profiles for documents intended
for broad groups of user agents, for instance, separate document
profiles for user agents running on desktops, television,
handhelds, cellphones and voice user agents. Document profiles
provide a basis for interoperability guarantees. If an author
develops content for a given profile and a user agent supports
the profile then the author may be confident that the document
will be rendered as expected. The requirements for document
profiles are analyzed.
Status of this document
This is a public W3C Working Draft for review by W3C members
and other interested parties. It is a draft document and may be
updated, replaced, or obsoleted by other documents at any time. It
is inappropriate to use W3C Working Drafts as reference material
or to cite them as other than "work in progress". A list of
current public W3C working drafts can be found at http://www.w3.org/TR.
This document has been produced as part of the W3C HTML Activity, but
should not be taken as evidence of consensus in the HTML Working
Group. The goals of the HTML Working Group (members only) are
discussed in the HTML Working Group
charter (members
only).
Please send detailed comments on this document to www-html-editor@w3.org. We
cannot guarantee a personal response, but we will try when it is
appropriate. Public discussion on HTML features takes place on the
mailing list
www-html@w3.org.
As vendors introduce new versions of user agents, content
developers have had to get to grips with a variety of differences
between user agents from the same vendor as well as the larger
differences between vendors. The World Wide Web Consortium plays
a stabilizing role through the development of standards, for
instance, HTML 3.2, HTML 4.0 and more recently XHTML 1.0.
Standards act as beacons for vendors, lighting the path towards
greater interoperability. Investment in existing code together
with the desire to innovate and differentiate act as pressures to
veer away from the light. The end result is that content
developers will be forced to continue to cope with
variations.
The range of user agent platforms is rapidly expanding to
include television sets, handheld organizers, cell phones, in-car
systems and regular phones. Each of these platforms presents
different capabilities. Viewing distances for television sets are
much greater than for desktop or notebook computers, reducing the
legibility of text. In addition saturated colors tend to bleed
and need to be avoided. Handheld devices have reduced resolution
and limited color capabilities. Cell phones are even more limited
with display resolutions of as little as 4 lines of 12
characters. Voice user agents substitute speech recognition for
keyboards, using synthetic or pre-recorded speech for output.
Users are likely to expect to be able to access Web services
from wherever they are and at any time - from home, on the move
or in the office. This will encourage content developers to reach
out to as wide an audience as possible, on as many kinds of user
agents as practical. One approach is to develop content
separately for each of the dominant platforms. Another is to
develop content in a way that lends itself to automatic
transformation to specific platforms. This approach emphasizes
the separation of content, structure and style. The
transformation can be done at authoring-time, by the originating
server, proxy server or in the user agent itself as
appropriate.
Document profiles offer a means to characterize the features
appropriate to given categories of user agents. For instance, one
profile might include support for style sheets, vector graphics
and scripting, while another might be restricted to the tags in
HTML 3.2. Document profiles can be used by servers to select
between document variants developed for different user agent
categories. They can be used to determine what transformations to
apply when such variants are not available. Content developers
can use document profiles to ensure that their web sites will be
rendered as intended.
Web content developers can manage differences in user agent
capabilities by developing several variants of their content,
where each variant is tuned to the capabilities of a particular
class of user agents. If the server is able to obtain information
about the capabilities of a given user agent, then the server can
select the variant of the Web content most appropriate to that
user agent. If a suitable variant isn't available it may be
practical to apply a transformation to generate one.
In the figure above, the client is either a user agent or a
proxy server. The client's capabilities describe hardware and
software constraints, such as display size, multimedia support,
support for scripting and style sheets etc. The client's
preferences reflect user settings. The client transmits these to
the server. The W3C Note on CC/PP
describes ways in which this can be realized. Additional
information is available from the IETF content negotiation
working group [CONNEG].
The server has access to the content. The request from the
client includes a name that the server can use to identify one or
more documents. Each document is associated with a document
profile. The server compares the client capabilities to the
document profiles to find a good match or to identify which
document to use as a basis for transformation to meet the
client's request.
This framework is consistent with a broadcast scenario in
which there is a uni-directional link between the transmitter and
receiver.
In a one-way scenario, a proxy server at the transmitter may
model the capabilities of the receivers and negotiate on the
receiver's behalf. In this scenario, the transmitter uses the
appropriate descriptions of the receiver capabilities in its
requests to the Web sites. As in the previous case, the
transmitter may transform the content to meet the capabilities of
the receiver or this may occur at the content server.
This capability is particularly useful in television and
mobile environments in which a return channel from the client is
either not available or desirable.
Further work is needed in four areas:
- Client capabilities and personal preferences
- Content negotiation -- the details of how capabilities and
preferences are conveyed in the request
- Document profiles -- describing groups of documents
- Document instance profiles -- a document might declare that
it is a member of one profile, but it only uses a subset of the
profile, making it a valid variant for other profiles
- Transformation of documents from one profile to another
Note: Preferences for modules
and attribute values etc. need to be treated as part of
formalization of client capabilities and personal preferences,
rather the document profiles which provide a declarative
description of a group of documents.
This section identifies a preliminary list of
requirements for further work on document profiles as distinct
from client capabilities and personal preferences. Please note
that this is work in progress and should not be taken as evidence
of consensus in the HTML working group. No significance should be
attached to the order in which the requirements are listed.
The requirements are partitioned into two categories: (1)
functional requirements, and (2) design requirements. Functional
requirements represent the needs of the content authoring
community that will use the profiling solution. These needs are
fundamentally independent of how that solution is implemented.
Design requirements represent the needs of the tools community
that will implement the profiling solution (or parts thereof).
These needs are may drive requirements that go beyond the
solution at hand, including requirements related to how the
profiling solution integrates with other problems in this problem
space.
Note: This document follows the
decision by the HTML working group to deliberately omit
consideration of how these requirements can be met. Proposed
solutions will be discussed in a separate draft.
3.1. Functional Requirements
Functional requirements correspond to the capabilities of
document profile system, and not how the solution is realized.
These requirements typically related to what features can be
found in the document profiles.
3.1.1. Content developers shall have a simple means of
associating a document profile with a document
There shall be a means for content developers to associate a
document with a document profile without having to specify the
features of that document profile.
A possible solution to this requirement is to allow content
developers to associate a name or location of the document
profile.
3.1.2. Content developers shall have a simple means of
describing a document profile in terms of features found in that
profile
Content developers may wish to provide more detail than a
simple document profile name. This is important if the name of
their document profile is not readily recognized (if it is new,
for example) or if their document profile is a variant of a
recognized document profile.
Document profile names must be unique.
The most obvious idea is to describe profiles as a list of
feature names, for instance, the names of modules, image formats,
scripting, and style sheet languages. Specific requirements for
some of these are spelled out below.
The semantics of features may be defined by binding them to
existing standards, or via additional information supplied as
part of an extended profile definition, or accessible via
registries.
In practice, in a flat name space, the number of names can get
out of hand. As a result, it is desirable to introduce a
hierarchical structure into feature names where each level in the
hierarchy scopes the name space for the next level of detail. One
possible approach for this is described in [RFC2506].
3.1.3. Content developers shall have a means of describing
exceptions to the general case (non-critical)
If profile features are described in purely atomic terms it
may be necessary to decompose the features into a lot of
subsidiary features. The ability to specify exceptions makes it
practical to use a higher level description together with the
exceptions, leading to much shorter descriptions. These
exceptions may be additive or subtractive.
The draft modularization proposed for XHTML includes over 20
modules. If you wanted to define a profile omitting just one of
these modules, an additive description would involve some 20
modules, compared with 2 for the case where you can state
exceptions.
Exceptions can take several forms, for instance, the ability
to add a new element, or to add a new attribute to a given
element, or a new attribute value to a given attribute. When
exceptions act to override some property, that is to take away an
element, attribute or value, things become more complex.
One possible approach is to provide an algebra for adding and
subtracting modules as a basis for describing document profiles,
where the document syntax is defined by reference to a DTD or XML
schema specifying the combined effect of the modules. This
approach delegates the effort of combining the DTDs or schemas
for each module to the author of the profile.
Note: The ability to deal with
inheritance and set subtraction could provide the basis for
formalizing the effects of module algebra on document syntax.
Dave Raggett has studied ways to achieve this using assertion
grammars.
3.1.4. Content developers shall have a means of describing
alternative features in a document profile
A document profile might specify that image/gif
or image/png support is required, but it is not
necessary for the client to support both features. There are
several features within the HTML, SMIL, and CSS that provide a
means for content developers to specify alternative content:
nesting object elements,the switch
element, and @media types. This means that a
document profile may indicate that it requires one feature or
another feature, but it is not necessary for the client to
support both features.
3.1.5. Content developers shall have a means of expressing
constraints on linked data formats
The need to say which image formats authors can rely on, e.g.
image/gif, image/jpeg, and
image/png. In addition, the profile may need to express
constraints on whether animated gifs are allowed, what optional
features of image/png are supported, and the maximum
file size permitted (e.g. < 20K).
3.1.6. Content developers shall have a means of expressing
detailed interoperability constraints for scripts and style
sheets
The variations in support for scripting and style sheets
across user agents and platforms cause real problems for content
developers. Document profiles need to be able to specify
constraints on the scripting language and interfaces, and on
which style properties and values are supported on what elements.
The capability to express these constraints would make it easier
to develop transformation tools. For instance, allowing content
developers to create content using a clean set of style
properties with automatic transformation into markup tuned to the
vagaries of particular user agents.
Note: Work on the DOM and on
modularizing CSS will go some of the way to alleviate the problem
this requirement addresses, but the weak conformance requirements
for CSS and scripting (as a whole) means that this problem won't
go away altogether.
3.1.7. Content developers shall have a means of expressing
expectations of user agent capabilities
A document profile may make certain assumptions about user
agent capabilities. By expressing these in the profile, servers
can use information about user agent capabilities as a basis for
selecting content with matching assumptions.
Examples include assumptions about display resolution, and
multimedia support; support for Java and 3D graphics; support for
particular character sets, e.g. for Kanji, and support for
cookies. These requirements need to be expressed in the same
vocabulary used to describe user agent capabilities, e.g. see
W3C's work on Client Capabilities and Personal Preferences [CC/PP].
3.1.8. Content authors shall have a means of defining
profiles for documents with controlled extensibility as will be
permitted by the XML Schema language
Sometimes it is impractical to provide a closed definition for
a group of documents. For instance, where the set of elements
representing business procedures is evolving over time, it may be
impractical to specify a frozen set of elements. The document
schema needs to be able to indicate where the content model or
attributes are open-ended and in what way. This requirement is
needed to cater for situations where you have partial knowledge,
but can also be exploited as a mechanism for dealing with forward
compatibility.
3.1.9. Content authors shall have a means of expressing
distribution rights based on user agent support for features of
the profile
A related point is the requirement to give authors control
over how user agents treat unknown elements and attributes. For
instance, should the user agent attempt to render the content of
an unknown element or not. This situation arises when a server
gets a request for a document from a user agent for which the
server can't provide a version of the document with a matching
profile.
The server can choose either to fail the request or to deliver
a document which the user agent may not be able to fully render.
The content author wishes to have some assurance in such cases
over how the user agent treats elements it doesn't understand.
Consistent treatment of this is essential to controlled evolution
of the Web. Without such a mechanism, content developers may feel
unable to deploy new features.
Note: The HTML working group
spent considerable time on this issue at the Amsterdam meeting in
May. W3C's SMIL specification includes support for this
feature.
3.1.10. Content developers shall have a means of expressing
how long the document profile may be cached
If software agents have to download profiles each time they
get a request to for a document, the response time will suffer.
As as result, agents will want to cache profiles. The requirement
is for an ability to specify an expiry date/time after which the
cached copy must be refreshed. In HTML documents, authors can
specify this via the meta element. If profiles are specified in
other than HTML, then another mechanism is needed to meet this
requirement.
3.1.11. Content developers shall have a means of expressing
attribution and copyright information
Additional information about who has defined the profile and
when.
3.1.12. Content developers shall have a means of expressing
required protocols
Content developers need to be able to specify if using a
document (or executing a script or resource associated with the
document) requires the use of a particular protocol (such as an
SSL connection).
3.2. Design Requirements
Design requirements correspond to how the solution is
implemented and how it will be used, independent of the features
found within the document profile solution.
3.2.1. The design shall support lightweight testing of two
profiles for equality
This is needed to support efficient run-time selection of
documents belonging to a given profile.
3.2.2. The design shall support lightweight testing of a
client's capabilities and preferences against a document's
profile.
To provide support for matching a client's capabilities and
preferences with the profiles of variant documents.
3.2.3. The design shall support machine readable
profiles
This is needed so that servers can autonomously perform
content selection in response to a request. In addition, this is
needed to support transformation agents.
3.2.4. The design shall specify document syntax by reference
to external definitions
This requirement is needed to decouple work on profiles from
that on document syntax. This will allow W3C to develop a
specification for XHTML document profiles independently of work
on XML Schemas. It will enable profiles to use XML 1.0 Document
Type Definitions or XML Schemas. The expectation is that over
time XML Schemas will supplant DTDs.
3.2.5. The design shall support formal verification that a
given document conforms to a profile
This is needed when content developers are unsure of
themselves or their tools. It necessitates a formal definition of
the profile that can be used to automate the testing of whether
or not the document conforms to the profile. This can be reduced
to verifying that the document conforms to syntax and semantic
constraints defined by the profile.
3.2.6. The design shall support multiple XML name spaces
Document profiles need to be able to describe documents
including elements from more than one name space, and the means
to verify that such documents conform the the profile. It is
strongly desirable that any such solution does not prescribe the
prefixes used for each name space.
3.2.7. The design shall support a human readable description
of the profile
Which can be shown to content developers to aid their
understanding of the purpose of the profile and appropriate ways
to create content for it.
3.2.8. The design shall support reference to specifications
and documentation defining a document type for the profiled
documents
A common means to express the name and/or location of
available specifications or documentation about the document type
being profiled. This is similar to 'Human readable description of
the profile' but is called out as a separate requirement to
ensure that links to specs and documentation are treated in a
very consistent and predictable manner.
3.2.9. The design shall use XML or RDF
The idea here is to avoid creating radically new formats by
constraining profiles to be expressed in XML or RDF.
The work on client capabilities and personal preferences is
represented in RDF, making it attractive to consider use RDF for
document profiles [CC/PP].
The way profile information is structured in the profile
document should be such that the effort is minimized to map
between a profile document and a content negotiation session
using the same data. This applies to both the features in the
profile and the algebra combining/negotiating them [CONNEG].
3.2.10. The design shall support a uniform way in which to
extend profiles
It should be easy to extend profiles with new kinds of
constraints as the need for these emerges.
3.2.11. The design shall support a means of specifying
document profile information inside the document.
content developers should have a simple means of specifying a
document profile and keeping that profile information tightly
coupled to a specific document.
3.2.12. The design shall support a means of specifying
document profiles external to the document
content developers should be able to specify a document
profile external to a document so that it may be readily reused
by other documents. This is needed to simplify the management and
maintenance of profiles shared by large number of documents.
Furthermore, the size of profiles may make it impractical to
incorporate them directly in documents.
3.2.13. The design shall support including document profile
information in a request to a server
To enable servers to identify content appropriate to each
client, the request must be able to include information that can
be used to find matching document profiles. This requirement acts
as a constraint on the representations of client
capabilities/preferences and document profiles.
The Web is dependent on the TCP/IP network protocols. When
opening an HTTP connection (using TCP/IP) the size of the initial
request has a considerable effect on the number of round trip
times needed for the response to be received by the requestor.
For optimum performance the request should fit into a single
packet. This places a premium on reducing the size of the profile
description sent as part of an HTTP request.
Note: This is an important
consideration when defining client-server protocols. If you want
to know more, Jim Gettys can explain this in great detail!
3.2.14. The design shall support in-place or linked
assertions
In much the same way as a file user agent allows folders to be
collapsed or expanded, the document profile should allow the
profile author to explicitly include a particular assertion, or
to include a link to it. In this way an otherwise identical
profile could be expressed explicitly, or as a list of titled
links, or some mix of both (the choice would depend upon
environmental or application considerations).
3.2.15. Profiles that are embedded in the document shall be
accessible through the Document Object Model
If the document profile information within the document, it
should be accessible through DOM interfaces.
3.2.16. The design shall support referencing resources
indirectly
The ability to express the name and/or location of a resource
resolution authority, such as a catalog file or name to resource
resolution server.
The editors wish to thank Murray Altheim and Håkon Wium
Lie for their feedback on earlier versions
-
[CC/PP]
- "Composite Capability/Preference Profiles (CC/PP): A user
side framework for content negotiation", F. Reynolds, J. Hjelm,
S. Dawkins, S. Singhal, 30 November 1998.
This document describes a method for using the Resource
Description Format (RDF) to create a general, yet extensible
framework for describing user preferences and device
capabilities. Servers can exploit this to customize the service
or content provided.
Available at:
http://www.w3.org/TR/NOTE-CCPP
-
[CONNEG]
- The IETF Content
Negotiation (conneg) Working Group which has defined a number
of RFC's relevant to document profiles.
-
[RFC2396]
- "RFC2396: Uniform Resource Identifiers (URI): Generic
Syntax", T. Berners-Lee, R. Fielding, L. Masinter, August
1998.
This document updates RFC1738 and RFC1808.
Available at:
http://www.ietf.org/rfc/rfc2396.txt
-
[RFC2506]
- "RFC2506: Media Feature Tag Registration Procedure", K.
Holtman, A. Mutz, March 1999. This is in the IETF category of
"Best Current Practice"
Available at:
http://www.ietf.org/rfc/rfc2506.txt
- [XML]
- "Extensible Markup Language (XML) 1.0 Specification", T.
Bray, J. Paoli, C. M. Sperberg-McQueen, 10 February 1998.
Available at:
http://www.w3.org/TR/REC-xml
-
[XMLNAMES]
- "Namespaces in XML", T. Bray, D. Hollander, A. Layman, 14
January 1999.
XML namespaces provide a simple method for qualifying names used
in XML documents by associating them with namespaces identified
by URI.
Available at:
http://www.w3.org/TR/REC-xml-names
