This approach has benefits as well as drawbacks. The benefits are that the XForms specification was completed more quickly, and without host language dependencies that otherwise might exist. The primary disadvantage is that more work needs to be done to actually integrate XForms with XHTML, SVG, or any other language.
Another W3C specification, Modularization of XHTML, provides a framework in which XHTML, or any other combination of XML-based languages, can be mixed and matched in order to provide a combined document type. Such combinations can take advantage of specific language features; for example, in XHTML a non-rendered head section can contain the XForms Model, and in SVG, a foreignObject element can enclose individual form controls.
Any document that uses XForms will necessarily be a combined document type, involving multiple XML namespaces. Such compound documents are still largely uncharted territory in the realm of W3C specifications, which leads to several headaches. For one thing, XML has the concept of attributes of type ID, specifying a document-unique value. Unfortunately, the id-ness of the attribute needs to be declared in a DTD or some kind of schema, which can only occur at the top of the overall document, not at the point where a subdocument starts. DTDs in general are poorly suited to validation, so until further work is done within the W3C, some XForms documents will have to suffice with being simply well-formed.
Although often scorned by developers, XML namespaces are a fact of life, particularly for W3C specifications. XForms elements conforming to the final W3C Recommendation are defined in a namespace of http://www.w3.org/2002/xforms. Other specifications could, in theory, include all the XForms elements in their own namespace, though this seems unlikely for official W3C specifications. Examples in this book show a mixture of both approaches.
One glimmer of hope is a recurring proposal for an attribute named xml:id, which would be recognized as having id-ness without a separate DTD or Schema. In examples throughout this book, any attributes named id will be considered to have been appropriately declared to be unique identifiers.
In a similar category is an attribute usually named class, which serves as a hook for attaching style sheets. As used throughout this chapter, the host language is responsible for defining this attribute and attaching it to the XForms elements.