This document is a "wish list" of enhancements to and concerns about
the DOM that were not addressed in Level 1, Level 2, or Level 3. Note
that the DOM Working Group has not committed to work on most of these
If you know of something that should be included here, please let us
know so we can update the document. If you can provide a summary, and
citations of past discussion, that would help the DOM Working Group a
Comments on this document are invited and are to be sent to the public
mailing list email@example.com. An
archive is available at http://lists.w3.org/Archives/Public/www-dom/.
This document has been produced as part of the W3C DOM Activity. The authors
of this document are the DOM WG members.
1. Deferred major functions
1.1 Views, Formatting
As raised by the CSS discussions. There may be multiple views of a
document; access to data for each view. (Editor, for example, may have
source and tree and rendered views active simultaneously. Might be aural
etc.) Batching of events and minimizing reformatting costs plays into
this. Also, accessing formatting results of applying style,
after rendering (but not including the render
itself). Presentation-specific. Partly a browser/engine-standardization
issue; see what we've got that we can agree on. Browser users looking for
standards beyond DOM Level 0. WAI wants selections/highlighting ideas,
view-specific computed values from CSS (which is why abstract View went
into CSS support -- but not Actual, i.e. final-rendered, values). Note
that the XSL folks also want us to start incorporating XSL support.
Note: see also Document
Object Model Level 3 Views and Formatting.
1.2 Transactions, batching of operations
E.g. roll-back if an operation fails. Database-style (failure) and/or
editor-style (rollback). May have to be able to handle complex compound
operations; may need to allow users to aggregate on top of that.
1.3 XSLT stylesheets and/or transformation support
Members of the XSL Working Group has strongly requested that we
support XSL at least to the degree that we do CSS. On a simple level that
isn't hard. It becomes much more complex if we attempt to allow mapping
nodes in the generated Document back to nodes in the source, for
selection and the like.
1.4 Repository interface
There is a significant amount of interest in providing tools to manage
a group of documents within a repository. There is not yet a clear
consensus on what a repository is or whether the DOM is the right place
to define an API for accessing it. Jeroen van Rotterdam has proposed the
following characteristics; they are included here for discussion, but the
DOM WG has neither endorsed or rejected them. For example, one of the
open questions in defining a repository is whether it contains "DOM
Documents", XML documents, or simply documents which may be
- A Repository should be able to contain several DOM Documents.
- A Repository should be able to contain a collection of Repositories
which is a recursive structure much like directories in an operating
- A repository should be able to retrieve, add, delete or update DOM
- A repository should be able to retrieve, add. delete or update its
- A repository should be able to handle checking out and checking in
of DOM Documents
- A repository should be able to hold a collection of DTD
- Every Document which is valid according to a DTD should be able to
access its DTD from the collection of DTD interfaces of its
corresponding Repository or any ancestor Repository.
- Repositories should be part of the DOM tree structure.
- Query languages should be able to execute their queries relative to
a Repository node.
1.5 Query Language interface
More input desired from XML Querying Working Group. Known issues:
query representation (string or object), response representation
(iterator/treewalker?), and whether querying is a DOM API or a more
general API that XML models, including DOMs, might implement. See also
the "XPath Support" item later.
1.6 Multi-threading support
Old description: "Subclassing of objects is not supported at level
1. Higher order locking will not be included in level one. Question of
how to have some people with read access at the same time as others have
write access while maintaining predictable results across
implementations. Minutes of 19971003 talk about handling concurrency
problems in level two."
requirements (October 9, 1997 version) document: "an external
security API will be provided after level one." Unclear whether
this means digital signatures, protecting an in-memory DOM fragment from
being written out to storage, controlling document-to-document scripting
access, or other.
1.8 Additional event sets
Sense-of-the-group is that we'd like others to design these, but we
may want to "bless" some of the most critical ones.
2 Individual Issues and Enhancements
2.1 Lightweight DOM
Sometimes called "server DOM", this is intended to be a "core of the
core" subset, reducing power but gaining performance/efficiency.
- Its value depends in part on whether full DOM can be
clever/efficient enough. "Unless order of magnitude improvement over
naive (at least) DOM, may not be worth considering."
- The challenge is that there seem to be many different overlapping
subsets. For example, palmtops may also want a reduced model but with a
different batch of functions. Some (Perl) implementors have asked
whether NodeLists, and even NamedNodeMaps, could be made static
snapshots rather than live accessors. And so on. These domains are not
yet well defined and seem to be a morass of special interests.
- One solution might be to break core into hasFeature sets. It's
unclear whether the divisions will always be clean ones, or whether
people would really want to deal with that fine a granularity. If too
much of the DOM becomes optional, interoperability suffers.
- Do we also need to represent "this works, but performance may be
far below what you would expect from a "full" DOM? (Contrived example:
get-parent implemented by search downward from root nodes.)
- Seems to need an Editorial Team to analyze requirements and
opportunities. That may make this a "deferred function" rather than a
2.2 isReadOnly test
It might be useful to have the opportunity to test whether a node is
currently readonly or not, without having to attempt to modify it and
catch the resulting exception or modify it back. Proposed solution:
adding an isReadonly flag to the Node interface.
2.3 Read-only DOM
There have been requests for a way to make part or all of the DOM tree
read-only. One use case would be when an immutable data structure wishes
to permit users to view its contents via a DOM API. Another would be if a
standard DOM was to be used to present a document that the users should
not be permitted to alter. The latter state might be either transient or
For example: XSLT's extension mechanism wants to expose the source
document via the DOM API, but does not want to permit extensions to alter
that document... even if the input was provided as a read/write DOM.
- This can be viewed either as a kind of "Lightweight DOM" (which
see), or as one instance/application of "data locking" (see
multitasking), depending on whether it's inherent in the implementation
or switchable via an API.
- DOM Level 1 defined the concept of read-only nodes as children of
Entity References. Making other nodes/trees read-only should not
involve much new behavioral definition beyond deciding when and how
that status can be set , cleared, or queried. Note that we would _not_
want to allow an EntityReference's contents to be set read-write!
- If we define a setReadOnly() operation (or equivalent), then
permanently read-only DOMs can simply pretend that this method was
called before the DOM was made available to the user's code.
(Read-only DOM implementations could almost claim to be compliant now,
since we state when a node is readonly but not when it
- ISSUE: We haven't decided how normalize() behaves when faced with
an unnormalized readonly subtree. Throw an exception and leave the
document unchanged? Exception but fix all read/write nodes? Exception
and only fix up to the exception point? Silently fix only read/write
nodes? Fix even the readonly nodes?
- ISSUE: Is this a read-only Document (ie, on doc-by-doc basis, maybe
node-by-node), or a read-only DOM? If the latter, then arguably the
node factory methods should throw exceptions; if the former, only
attempting to insert nodes should object. This also has implications
for the statement that cloneNode() produces a mutable copy.
- Some folks really don't want to have to implement mutable nodes and
node factories at all. The proper solution for that, architecturally,
appears to be to build another layer into the DOM -- a read-only
superclass which is subclassed to produce the original read/write
DOM. That's a nontrivial change, and there has not been complete
agreement over what functions belong in each layer.
suggested that higher-level objects (NodeLists, Ranges, and the like)
may want to be able to be made immutable independently of the nodes
they refer to -- eg, to support a non-live version of these views as a
cached query result (cf to his post for a proposed approach).
- The general concept seems to be reasonable. But there is some
creeping featurism setting in, and there were enough open questions to
prevent it from immediately going into DOM Level 2. The challenge is
agreeing on exactly how "a read-only DOM" is defined, and deciding
whether a single solution will indeed be widely useful.
2.4 Enhance Traversal to merge adjacent Text Nodes
Ability to traverse a normalized view of an un-normalized
document. Same issues as "Text Node Amelioration"
- Should it really merge/normalize the adjacent text nodes, or
simulate a single node?
- How does this interact with filtering?
- Should this be able to "normalize" through CDATASections?
2.5 Should NodeIterator be cloneable>
Since NodeIterator doesn't have the ability to setCurrentNode, saving
and reusing an iteration state is difficult; one has to create a new
iterator, then step it until it reaches the right place. The suggestion
has been made to add NodeIterator.clone().
- This may be expensive if the NodeIterator is doing something fairly
fancy under the covers, such as running a query engine. But it should
be no more expensive than the step-until-synchronized approach.
- Some have put this in terms of NodeIterators produced by other than
our factory methods. As the Traversal team anticipated, folks are
trying to generalize the concept to handle richer kinds of query and
2.6 Pseudo-attributes on Processing Instructions
While the value of a processing instruction is defined by XML as being
an unstructured string, one approach to using that to contain multiple
parameters is to borrow XML's Attribute syntax. If this turns out to be a
common usage, the DOM might want to facilitate it.
- What should happen with PIs that don't follow this syntax, or
instances which intended to but didn't do so properly? We'd need to
make the calls smart enough to report parsing errors.
- Do we support only XML-style attrs, or HTML-style unquoted
- Should this allow setting pseudo-attributes as well as retrieving
them? If so, do we attempt to emulate Attr's structural model, or are
pseudo-attrs text only?
- Related to that last: What do we do with Entity References?
- Current sense of the WG is that this should be left in
2.7 Attr.getValue() containing unresolved EntityReference
As of DOM2, what should happen in this case is undefined. The
definition of Attr.getValue() refers to the "value" of the EntRef -- but
EntRefs have no node value.
- We could throw an exception. Problem: SIGNATURE CHANGE?
- We could have attr treat undefined EntRef as empty. Problem: Not
getElementsByID() has been added to DOM Level 2, but evaluating this
depends on DTD support (non-DOM, in Level 2) to determine what attributes
are or aren't IDs. It was suggested that for non-validated documents or
other situations where the document's author didn't declare the attribute
as ID, we offer a lookup by attribute value (falling back on the
possibly-optimized ID retrieval when possible). There is some sense that
this more general search might be independently useful even if it can't
- Some DOMs are providing implementation-specific versions now.
- The generalized version, unlike IDs, may return multiple nodes.
There isn't consensus on how to do so. Some strongly prefer Iterators,
suggesting that this is just a Filter and that implementations could
recognize a special Filter and optimize the Iterator into a
lookup. Others argue that iterators aren't Core and don't want the
prerequisite, leading them to favor NodeLists. Some say "Our real goal
is IDs, and general querying will be addressed eventually, so for now
just return the first match and let it go at that.
- Note that XML Schemas introduce "keys", another way of uniquely
identifying a portion of a document.
2.9 Allow event to suppress capture phase
Level 2 allows us to turn off bubbling of events (by setting "bubbles"
to false), but there is no corresponding capability to suppress capture
on an event-by-event basis. In the public list, John Ky has suggested
that there are times when this would be desirable. While it is certainly
possible to simply not register any Capturing listeners, and a
sufficiently clever implementation may be able to avoid spending cycles
on the Capture phase unless listeners exist which need it, it may be
useful to be able to say "This specific event really is intended
specifically for the stated Event Target and should not be captured."
2.10 Make TreeWalker attributes writable
Since TreeWalker follows strict current-node semantics, it could
arguably be made safe to alter its filter, whatToShow, or even root node
properties. Miles suggests this as an alternative to
generating/maintaining multiple TreeWalkers. We currently recommend
against this change; it seems to have significant risk for minimal
- The assumed efficiency gains are highly dependent upon the exact
details of how the DOM is implemented. You'd avoid constructing a new
TreeWalker object, but might wind up paying significantly more than
that inside the DOM.
- One example of that: Implementations which generate specialized
implementations of Traversal objects -- optimized for particular
predefined filters, perhaps as the result of a complex query -- would
find this feature hard to support. The best approach we've found would
involve proxying, which would impose significant performance
- In fact, one can already change filtering conditions in
mid-traversal, by writing a filter which can change the criteria used
for its tests. If the nodeType test is moved into the filter and
SHOW_ALL is used, that too can be altered on the fly. We recommend
against doing so -- indecisive filters can be hard to debug and may
surface portability problems -- but it's theoretically possible.
Quick reset to initial/final states. See last paragraph of http://lists.w3.org/Archives/Member/w3c-dom-ig/1999Aug/0088.html
2.12 Diamond inheritance proposed
Architecturally, many (not all) of the additional interfaces added in
DOM Level 2 should inherit from the base classes that will implement them
-- for example, DocumentTraversal should inherit from Document -- to
document and enforce that association, and to avoid unnecessary
typecasting. This introduces "diamond inheritance" (B is-an A, C is-an A,
D is both a B and a C), which was a problem for some object-oriented
- Is diamond inheritance safe for all modern languages likely to
implement the DOM, and for all implementations thereof? Probably, these
days. But we aren't 100% confident. If not, it probably doesn't belong
in our architecture
- There was a concern that CORBA bindings might have trouble with
typecasting. Latest evidence is that they shouldn't, but that casting
of this sort may be a higher-overhead operation. On the other hand,
this is not an operation that will be performed frequently.
- Possible alternative: publish separate architectural and
CORBA-binding IDL, and only close the diamond in the latter. This has
some risk of confusing the readers.
2.13 OMG XML RFP
The OMG wants a way to represent XML content as a datatype... with or
without "data binding" to application-specific datatypes. The request,
and proposals, should be available at
At least one proposal based on the DOM API has been submitted.
Essentially, this provides an IDL binding for the DOM which emphasizes
access to local documents.
2.14 I18N Indexing
The DOM is currently expressed as storing data in 16-bit
"units". Unicode permits characters longer than this, via diacritics and
so on. The I18N folks requested that the DOM provide mapping between
character-offsets and unit-offsets for text we have stored. We seem to
agree that there's a legitimate need to do this, but have doubts about
whether it's an operation that should be defined by and within the
- Strong sense in DOM WG that this proposal was at the wrong
granularity. Converting counts would yield an expensive version of
substring, for example; you'd have to recount from the beginning,
rather than continuing the count from the last known position.
- Fixing it inside the DOM doesn't help applications much -- what
about info not currently stored in a DOM node?
- But there are some things that could be done with a string type
specifically designed to handle this, e.g. cache offset points to avoid
some of the repetitive scanning.
- Some opinion was expressed that I18N should define a UCSstring API,
which could address this properly, and which we could then incorporate
into our API as a returned type. (Perhaps "the preferred implementation
- Most recent proposal was to include non-normative language, passing
the buck to the binding's DOMString and showing the interfaces I18N
thinks it should provide.
2.15 Encoding support
The DOM is currently expressed as storing data in UTF-16. Some
annoyance has been expressed with the need to convert data from an
environment's native string encodings into this one, especially when
there may be multiple encodings within a country (EBCDIC and ASCII,
several Japanese encodings, etc.)
- Given that DOM Level 3 Serialization is implicitly going to have to
deal with encoding conversions, should we expose them as
- Or is this another "There ought to be an internationalized text
string datatype" point, passed to whoever eventually winds up dealing
with that problem?
- See the I18N Indexing issue for related discussion.
2.16 Non-Well-Formed Entities
Currently createDocument can only be used to create an XML document
with a single Document Element. Some applications want to use the DOM to
create a text document, or an external parsed entity, not contained in a
single root. Essentially, they would like to broaden the concept of
Use case: XSLT is now able to output things other than well-formed
XML. Our own spec, with non-XML bindings generated from XML, is an
example of that sort of processing. It would make a certain amount of
sense for the DOM to be able to capture XSLT's output even in this
- Currently available solution: Use a standard Document, build the
contents as children of an Element or DocumentFragment, and make the
serializer smart enough to know that it should discard this
- Proposal 1: modify createDocument so that it doesn't need a
documentElement. For example if that parameter is set to null, then no
documentElement is created. In this case, the doctype should be null
for documents containing text, comments, PIs, but may not be null for
documents containing elements. ISSUE: Document can't currently accept
all the intended children (text nodes, for example), so this is a
distinctly different mode of operation, and arguably a different kind
- Proposal 2: allow a stand-alone document fragment that doesn't have
an owner document and push the rest onto the serialization
module. ISSUE: Is this really significantly better than the current
solution of having creating a Document that you don't intend to
directly reference, creating a normal DocumentFragment from that, and
using the same specialized serializer?
- Proposal 3: add a createExternalParsedEntityContent method (or some
other name) which is like createDocument but doesn't have a
documentElement and can be used for text files or external parsed
entities with markup.
2.17 Visitor design pattern
We get periodic requests to implement the Visitor pattern [Design
Patterns, 1994, Gamma et al].
Visitor is a standard object design which allows you to execute a
unique method based on two object types. This is known as
"double-dispatching". The intent in this case would be to execute
different code based on the combination of a specific Node subtype and
a specific Operation type. (The Operation would correspond to the
In order to synthesize double-dispatching in a language that only
supports single-dispatching (e.g. C++, Java, Perl) a method must be
defined in the Operation interface which corresponds to each Node type,
and each Node must implement an Accept(Operation) method which invokes
the appropriate support method.
- Pro: Visitor provides a well-known idiom for defining an operation
to be performed on elements of an object structure.
- Con: Visitor is intended to address structures containing objects
with varying interfaces. Since the DOM tree is made up entirely of
Nodes, is it unclear that this capabiility is really advantageous in
- Con: The Visitor model makes adding new operations easy... at the
cost of making it hard to add new types/subtypes of objects. Quoting
James Melton: "If client code extended the Node hierarchy it would
break any existing [Visitor] implementation. Further, if the DOM was
extended to include a new Node subtype the Operator interface would
have to be extended and every Operator implementation would require
updating before the client could use the new DOM version."
- Con: And in fact, XML processing will often want to dispach on
namespace and node name as well as node type. If the nodes aren't
subclassed to that level, the Visitor's dispatch is only the first, and
possibly least, step in determining the node's semantic role and the
proper behavior of this Operation.
- Conclusion: Writing an explicit dispatch which examines the node
and switches appropriately for this Operation may not be as elegantly
object-oriented, but it accomplishes essentiallly the same goal, seems
to allow better extensibility, and our current best estimate is that
it's likely to yield better performance in most DOM implementations. So
at this time, we don't see significant value in adding Accept/Visitor
to the DOM API.
2.18 Error reporting
A list of error codes could be developed for USLs and other languages
that don't support exceptions
2.19 Meta data
"For level 1, just have all meta data be of type string/UTF-8 (for the
5 minimal things), but the type system must be extensible in the long
run. Level two will have more on meta data."
2.20 URis and systemIds
For nodes exposing a SystemId, there are cases where it needs to be a
fully resolved URI; and there are cases where it needs to be