Default Prefix Declaration

1. Disclaimer and backpointer

The ideas behind the proposal presented here are neither particularly new nor particularly mine. I've made the effort to write this down so anyone wishing to refer to ideas in this space can say "Something along the lines of [this posting]" rather than "Something, you know, like, uhm, what we talked about, prefix binding, media-type-based defaulting, that stuff".

This is an expanded version of a W3C QA Blog entry. It has no official standing, but is input into ongoing TAG discussion of the distributed extensibility issue.

2. Introduction

Criticism of XML namespaces as an appropriate mechanism for enabling distributed extensibility for the Web typically targets two issues:

1. Syntactic complexity
2. API complexity

Of these, the first is arguably the more significant, because the number of authors exceeds the number of developers by a large margin. Accordingly, this proposal attempts to address the first problem, by providing a defaulting mechanism for namespace prefix bindings which covers the 99% case.

3. Goal(s) of this exercise

There are two potential targets for the design presented here:

• XML itself, specifically a new version of XML namespaces;
• (The HTML serialization of) HTML5.

The former is a long-term target at best. As well as the well-known commitment to stability on the part of the XML community, there is the additional low-level technical problem that as things currently stand there is no mechanism for specifying XML namespace version information in an XML document.

HTML5 is then the primary target. This proposal could be seen either as an addition or as an alternative to the Microsoft proposal.

4. The proposal

Binding

Define a trivial XML language which provides a means to associate prefixes with namespace names (URIs);

Invoking from HTML

Define a link relation dpd for use in the (X)HTML header;

Invoking from XML

Define a processing instruction xml-dpd and/or an attribute xml:dpd for use at the top of XML documents;

Defaulting by Media Type

Implement a registry which maps from media types to a published dpd file;

Semantics

Define a precedence, which operates on a per-prefix basis, namely xmlns: >> explicit invocation >> application built-in default >> media-type-based default, and a semantics in terms of namespace information items or appropriate data-model equivalent on the document element.

5. Why prefixes?

XML namespaces provide two essentially distinct mechanisms for 'owning' names, that is, preventing what would otherwise be a name collision by associating names in some way with some additional distinguishing characteristic:

1. By prefixing the name, and binding the prefix to a particular URI;
2. By declaring that within a particular subtree, unprefixed names are associated with a particular URI.

In XML namespaces as they stand today, the association with a URI is done via a namespace declaration which takes the form of an attribute, and whose impact is scoped to the subtree rooted at the owner element of that attribute.

Liam Quin has proposed an additional, out-of-band and defaultable, approach to the association for unprefixed names, using patterns to identify the subtrees where particular URIs apply. I've borrowed some of his ideas about how to connect documents to prefix binding definitions.

The approach presented here is similar-but-different, in that its primary goal is to enable out-of-band and defaultable associations of namespaces to names with prefixes, with whole-document scope. The advantages of focussing on prefixed names in this way are:

• Ad-hoc extensibility mechanisms typically use prefixes. The HTML5 specification already has at least two of these: aria- and data-;
• Prefixed names are more robust in the face of arbitrary cut-and-paste operations;
• Authors are used to them: For example XSLT stylesheets and W3C XML Schema documents almost always use explicit prefixes extensively;
• Prefix binding information can be very simple: just a set of pairs of prefix and URI;
• Authoring with prefixed names is particularly appropriate in the HTML context, because it explicitly marks all vocabulary items which are not part of the embedding matrix language;
• It works for attributes as well as elements, where the default binding does not. Indeed somewhat surprisingly the HTML5 specification actually supports prefixed attributes in certain limited cases!

Provision is also made for optionally specifying a binding for the default namespace at the document element, primarily for the media type registry case, where it makes sense to associate a primary namespace with a media type.

6. Example

If this proposal were adopted, and a dpd document for use in HTML 4.01 or XHTML1:

<dpd ns="http://www.w3.org/1999/xhtml">
 <pd p="xf" ns="http://www.w3.org/2002/xforms"/>
 <pd p="svg" ns="http://www.w3.org/2000/svg"/>
 <pd p="ml" ns="http://www.w3.org/1998/Math/MathML"/>
</dpd>

was registered against the text/html media type, the following would result in a DOM with html and body elements in the XHTML namespace and an input element in the XForms namespace:

<html> <body>
 <xf:input ref="xyzzy">...</xf:input>
 </body>
</html>

7. Open questions and problems