XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (163 page)

I said that the processor looks in the schema for an appropriate element declaration, but where does it find the schema? It knows the namespace of the element name at this stage, so if a schema for this target namespace has been imported using

(see page 368), then there is no problem. Otherwise, the specification leaves things rather open. It recognizes that some processors are likely to have some kind of catalog or repository that enables the schema for a given namespace to be found without difficulty, and it allows this to happen where the implementation supports it. You can also create an
xsi:schemaLocation
attribute node on the element being validated, to provide guidance on where a schema document might be found. In other cases, the implementation is allowed to report an error.

If neither the
type
nor
validation
attribute is present, then the system behaves as if the
validation
attribute were present and had the value given by the
default-validation
attribute of the containing

element. If no default is specified at that level, the effect is the same as
validation=“strip”
.

XSLT does not provide any way to request validation of an element against a local element or type definition in a schema. The way around this is to request validation only when you create an element for which there is a top-level definition in the schema. This will then implicitly validate the whole subtree contained by that element, including elements that have local definitions in the schema. Alternatively, many locally declared elements make use of a globally defined type, and you can then use the
type
attribute to validate against the type definition.

Usage and Examples

In most cases, elements in a result tree can be generated either using literal result elements in the stylesheet, or by copying a node from the source document using

.

The only situations where

is absolutely needed are therefore where the element name in the result document is not fixed and is not the same as an element in the source document.

Using

rather than a literal result element can also be useful where different namespaces are in use. It allows the namespace URI of the generated element to be specified explicitly, rather than being referenced via a prefix. This means the namespace does not have to be present in the stylesheet itself, thus giving greater control over exactly which elements the namespace declarations are attached to. With a literal result element, all in-scope namespaces from the stylesheet are copied to the result document unless you exclude them using
[xsl:]exclude-result-prefixes
; this does not happen for

.

See Also

on page 254

on page 287

xsl:fallback

The

instruction is used to define processing that should occur if no implementation of its parent instruction is available.

Changes in 2.0

None.

Format

Position

is an instruction. It is generally used within a sequence constructor. However, some new XSLT 2.0 instructions (

,

) allow an

element as a child even though they do not contain a sequence constructor. This is to allow fallback behavior to be defined for use when these instructions are encountered by an XSLT 1.0 processor.

Attributes

None.

Content

A sequence constructor.

Effect

There are two circumstances where

can be useful:

• In a stylesheet that uses XSLT features defined in version 2.0, to indicate what should happen if the stylesheet is used with an XSLT 1.0 processor. For example, the XSLT 2.0 specification introduces the new instruction
  
  . If you want to use the
  
  instruction in a stylesheet, and also want to specify what an XSLT 1.0 processor that doesn't understand this instruction should do, you can define the required behavior using
  
  . XSLT 1.0 was carefully designed with extensibility in mind, so every XSLT 1.0 processor should implement this fallback behavior correctly even though the new XSLT 2.0 instructions were not defined at the time the processor was written.
  
• In a stylesheet that uses extension elements provided by a vendor, by the user, or by a third party, to indicate what should happen if the stylesheet is used with an XSLT processor that does not support these extensions.
  

If the

instruction is encountered in a sequence constructor that the processor can evaluate normally, it is ignored, along with its contents.

An instruction (as distinct from a literal result element) is an element that occurs in a sequence constructor and is either:

• in the XSLT namespace, or
  
• in a namespace designated as an extension namespace by its inclusion in the
  [xsl:]extension-element-prefixes
  attribute of the element itself or a containing element. This attribute must be in the XSLT namespace if its parent element is
  not
  in the XSLT namespace, and vice versa.