XSL Transformations (XSLT) Version 4.0

1.1 What is XSLT?

XSLT is a programming language designed principally to transform data.

A transformation in the XSLT language is expressed in the form of a stylesheet. A stylesheet is made up of one or more well-formed XML [XML 1.0] documents conforming to the Namespaces in XML Recommendation [Namespaces in XML].

A stylesheet generally includes elements that are defined by XSLT as well as elements that are not defined by XSLT. XSLT-defined elements are distinguished by use of the namespace http://www.w3.org/1999/XSL/Transform (see 3.1.1 XSLT Namespace), which is referred to in this specification as the XSLT namespace. Thus this specification is a definition of the syntax and semantics of the XSLT namespace.

The term stylesheet reflects the fact that one of the important roles of XSLT is to add styling information to an XML source document, by transforming it into a document consisting of XSL formatting objects (see [XSL-FO]), or into another presentation-oriented format such as HTML, XHTML, or SVG. However, XSLT is used for a wide range of transformation tasks, not exclusively for formatting and presentation applications.

A transformation expressed in XSLT describes rules for transforming input data into output data. The inputs and outputs will all be instances of the XDM data model, described in [XDM 4.0]. In the simplest and most common case, the input is an XML document referred to as the source document or source tree, and the output is an XML document referred to as the result tree. It is also possible to process multiple source documents, to generate multiple result documents, and to handle formats other than XML.

The transformation is achieved by a set of template rules. A template rule associates a pattern, which typically matches nodes in the source document, with a sequence constructor. In many cases, evaluating the sequence constructor will cause new nodes to be constructed, which can be used to produce part of a result tree. The structure of the result trees can be completely different from the structure of the source trees. In constructing a result tree, nodes from the source trees can be filtered and reordered, and arbitrary structure can be added. Because patterns can point to nodes according to typological features, a stylesheet can be applicable to a wide class of source documents that have similar tree structures.

Stylesheets are modular; they may contain several packages developed independently of each other, and each package may consist of several stylesheet modules.

[Definition: A stylesheet consists of one or more packages: specifically, one top-level package and zero or more library packages.][Definition: [Definition: A stylesheet consists of one or more packages: specifically, one top-level package and zero or more library packages.][Definition: [Definition: [Definition: A stylesheet consists of one or more packages: specifically, one top-level package and zero or more library packages.]

[Definition: For a given transformation, one package functions as the top-level package. The complete stylesheet is assembled by finding the packages referenced directly or indirectly from the top-level package using xsl:use-package declarations: see 3.4.2 Dependencies between Packages.][Definition: [Definition: For a given transformation, one package functions as the top-level package. The complete stylesheet is assembled by finding the packages referenced directly or indirectly from the top-level package using xsl:use-package declarations: see 3.4.2 Dependencies between Packages.][Definition: [Definition: [Definition: For a given transformation, one package functions as the top-level package. The complete stylesheet is assembled by finding the packages referenced directly or indirectly from the top-level package using xsl:use-package declarations: see 3.4.2 Dependencies between Packages.]

[Definition: Every package within a stylesheet, other than the top-level package, is referred to as a library package.][Definition: [Definition: Every package within a stylesheet, other than the top-level package, is referred to as a library package.][Definition: [Definition: [Definition: Every package within a stylesheet, other than the top-level package, is referred to as a library package.]

[Definition: Within a package, one stylesheet module functions as the principal stylesheet module. The complete package is assembled by finding the stylesheet modules referenced directly or indirectly from the principal stylesheet module using xsl:include and xsl:import elements: see 3.10.2 Stylesheet Inclusion and 3.10.3 Stylesheet Import.][Definition: [Definition: Within a package, one stylesheet module functions as the principal stylesheet module. The complete package is assembled by finding the stylesheet modules referenced directly or indirectly from the principal stylesheet module using xsl:include and xsl:import elements: see 3.10.2 Stylesheet Inclusion and 3.10.3 Stylesheet Import.][Definition: [Definition: [Definition: Within a package, one stylesheet module functions as the principal stylesheet module. The complete package is assembled by finding the stylesheet modules referenced directly or indirectly from the principal stylesheet module using xsl:include and xsl:import elements: see 3.10.2 Stylesheet Inclusion and 3.10.3 Stylesheet Import.]

2.1 Terminology

For a full glossary of terms, see B Glossary.

[Definition: The software responsible for transforming source trees into result trees using an XSLT stylesheet is referred to as the processor. This is sometimes expanded to XSLT processor to avoid any confusion with other processors, for example an XML processor.][Definition: [Definition: The software responsible for transforming source trees into result trees using an XSLT stylesheet is referred to as the processor. This is sometimes expanded to XSLT processor to avoid any confusion with other processors, for example an XML processor.][Definition: [Definition: [Definition: The software responsible for transforming source trees into result trees using an XSLT stylesheet is referred to as the processor. This is sometimes expanded to XSLT processor to avoid any confusion with other processors, for example an XML processor.]

[Definition: A specific product that performs the functions of an XSLT processor is referred to as an implementation.][Definition: [Definition: A specific product that performs the functions of an XSLT processor is referred to as an implementation.][Definition: [Definition: [Definition: A specific product that performs the functions of an XSLT processor is referred to as an implementation.]

[Definition: The term tree is used (as in [XDM 4.0]) to refer to the aggregate consisting of a parentless node together with all its descendant nodes, plus all their attributes and namespaces.][Definition: [Definition: The term tree is used (as in [XDM 4.0]) to refer to the aggregate consisting of a parentless node together with all its descendant nodes, plus all their attributes and namespaces.][Definition: [Definition: [Definition: The term tree is used (as in [XDM 4.0]) to refer to the aggregate consisting of a parentless node together with all its descendant nodes, plus all their attributes and namespaces.]

The output of a transformation consists of the following:

1. [Definition: A principal result: this can be any sequence of items (as defined in [XDM 4.0]).] The principal result is the value returned by the function or template in the stylesheet that is nominated as the entry point, as described in 2.3 Initiating a Transformation.[Definition: [Definition: A principal result: this can be any sequence of items (as defined in [XDM 4.0]).] The principal result is the value returned by the function or template in the stylesheet that is nominated as the entry point, as described in 2.3 Initiating a Transformation.[Definition: [Definition: [Definition: A principal result: this can be any sequence of items (as defined in [XDM 4.0]).] The principal result is the value returned by the function or template in the stylesheet that is nominated as the entry point, as described in 2.3 Initiating a Transformation.

2. [Definition: Zero or more secondary results: each secondary result can be any sequence of items (as defined in [XDM 4.0]).] A secondary result is the value returned by evaluating the body of an xsl:result-document instruction.[Definition: [Definition: Zero or more secondary results: each secondary result can be any sequence of items (as defined in [XDM 4.0]).] A secondary result is the value returned by evaluating the body of an xsl:result-document instruction.[Definition: [Definition: [Definition: Zero or more secondary results: each secondary result can be any sequence of items (as defined in [XDM 4.0]).] A secondary result is the value returned by evaluating the body of an xsl:result-document instruction.

3. Zero or more messages. Messages are generated by the xsl:message and xsl:assert instructions, and are described in 23.1 Messages and 23.2 Assertions.

4. Static or dynamic errors: see 2.12 Error Handling.

The principal result and the secondary results may be post-processed as described in 2.3.6 Post-processing the Raw Result.

[Definition: The term result tree is used to refer to any tree constructed by instructions in the stylesheet. A result tree is either a final result tree or a temporary tree.][Definition: [Definition: The term result tree is used to refer to any tree constructed by instructions in the stylesheet. A result tree is either a final result tree or a temporary tree.][Definition: [Definition: [Definition: The term result tree is used to refer to any tree constructed by instructions in the stylesheet. A result tree is either a final result tree or a temporary tree.]

[Definition: A final result tree is a result tree that forms part of the output of a transformation: specifically, a tree built by post-processing the items in the principal result or in a secondary result. Once created, the contents of a final result tree are not accessible within the stylesheet itself.] Any final result tree may be serialized as described in 26 Serialization.[Definition: [Definition: A final result tree is a result tree that forms part of the output of a transformation: specifically, a tree built by post-processing the items in the principal result or in a secondary result. Once created, the contents of a final result tree are not accessible within the stylesheet itself.] Any final result tree may be serialized as described in 26 Serialization.[Definition: [Definition: [Definition: A final result tree is a result tree that forms part of the output of a transformation: specifically, a tree built by post-processing the items in the principal result or in a secondary result. Once created, the contents of a final result tree are not accessible within the stylesheet itself.] Any final result tree may be serialized as described in 26 Serialization.

[Definition: The term source tree means any tree provided as input to the transformation. This includes the document containing the global context item if any, documents containing nodes present in the initial match selection, documents containing nodes supplied as the values of stylesheet parameters, documents obtained from the results of functions such as document, doc, and collection, documents read using the xsl:source-document instruction, and documents returned by extension functions or extension instructions. In the context of a particular XSLT instruction, the term source tree means any tree provided as input to that instruction; this may be a source tree of the transformation as a whole, or it may be a temporary tree produced during the course of the transformation.][Definition: [Definition: The term source tree means any tree provided as input to the transformation. This includes the document containing the global context item if any, documents containing nodes present in the initial match selection, documents containing nodes supplied as the values of stylesheet parameters, documents obtained from the results of functions such as document, doc, and collection, documents read using the xsl:source-document instruction, and documents returned by extension functions or extension instructions. In the context of a particular XSLT instruction, the term source tree means any tree provided as input to that instruction; this may be a source tree of the transformation as a whole, or it may be a temporary tree produced during the course of the transformation.][Definition: [Definition: [Definition: The term source tree means any tree provided as input to the transformation. This includes the document containing the global context item if any, documents containing nodes present in the initial match selection, documents containing nodes supplied as the values of stylesheet parameters, documents obtained from the results of functions such as document, doc, and collection, documents read using the xsl:source-document instruction, and documents returned by extension functions or extension instructions. In the context of a particular XSLT instruction, the term source tree means any tree provided as input to that instruction; this may be a source tree of the transformation as a whole, or it may be a temporary tree produced during the course of the transformation.]

[Definition: The term temporary tree means any tree that is neither a source tree nor a final result tree.] Temporary trees are used to hold intermediate results during the execution of the transformation.[Definition: [Definition: The term temporary tree means any tree that is neither a source tree nor a final result tree.] Temporary trees are used to hold intermediate results during the execution of the transformation.[Definition: [Definition: [Definition: The term temporary tree means any tree that is neither a source tree nor a final result tree.] Temporary trees are used to hold intermediate results during the execution of the transformation.

Unless otherwise stated, the term “tree” refers to a tree rooted at a parentless node: that is, the term does not include subtrees of larger trees. Every node therefore belongs to exactly one tree.

In this specification the phrases must, must not, should, should not, may, required, and recommended, when used in normative text and rendered in small capitals, are to be interpreted as described in [RFC2119].

Where the phrase must, must not, or required relates to the behavior of the XSLT processor, then an implementation is not conformant unless it behaves as specified, subject to the more detailed rules in 27 Conformance.

Where the phrase must, must not, or required relates to a stylesheet then the processor must enforce this constraint on stylesheets by raising an error if the constraint is not satisfied.

Where the phrase should, should not, or recommended relates to a stylesheet then a processor may produce warning messages if the constraint is not satisfied, but must not treat this as an error.

[Definition: In this specification, the term implementation-defined refers to a feature where the implementation is allowed some flexibility, and where the choices made by the implementation must be described in documentation that accompanies any conformance claim.][Definition: [Definition: In this specification, the term implementation-defined refers to a feature where the implementation is allowed some flexibility, and where the choices made by the implementation must be described in documentation that accompanies any conformance claim.][Definition: [Definition: [Definition: In this specification, the term implementation-defined refers to a feature where the implementation is allowed some flexibility, and where the choices made by the implementation must be described in documentation that accompanies any conformance claim.]

[Definition: The term implementation-dependent refers to a feature where the behavior may vary from one implementation to another, and where the vendor is not expected to provide a full specification of the behavior.] (This might apply, for example, to limits on the size of source documents that can be transformed.)[Definition: [Definition: The term implementation-dependent refers to a feature where the behavior may vary from one implementation to another, and where the vendor is not expected to provide a full specification of the behavior.] (This might apply, for example, to limits on the size of source documents that can be transformed.)[Definition: [Definition: [Definition: The term implementation-dependent refers to a feature where the behavior may vary from one implementation to another, and where the vendor is not expected to provide a full specification of the behavior.] (This might apply, for example, to limits on the size of source documents that can be transformed.)

In all cases where this specification leaves the behavior implementation-defined or implementation-dependent, the implementation has the option of providing mechanisms that allow the user to influence the behavior.

A paragraph labeled as a Note or described as an example is non-normative.

Many terms used in this document are defined in the XPath specification [XPath 4.0] or the XDM specification [XDM 4.0]. Particular attention is drawn to the following:

• [Definition: The term atomization is defined in [XPath 4.0] section 2.6.3 Atomization. It is a process that takes as input a sequence of items, and returns a sequence of atomic items, in which the nodes are replaced by their typed values as defined in [XDM 4.0]. Arrays (see 22 Arrays) are atomized by atomizing their members, recursively.] For some items (for example, elements with element-only content, function items, and maps, atomization raises a dynamic error.[Definition: [Definition: The term atomization is defined in [XPath 4.0] section 2.6.3 Atomization. It is a process that takes as input a sequence of items, and returns a sequence of atomic items, in which the nodes are replaced by their typed values as defined in [XDM 4.0]. Arrays (see 22 Arrays) are atomized by atomizing their members, recursively.] For some items (for example, elements with element-only content, function items, and maps, atomization raises a dynamic error.[Definition: [Definition: [Definition: The term atomization is defined in [XPath 4.0] section 2.6.3 Atomization. It is a process that takes as input a sequence of items, and returns a sequence of atomic items, in which the nodes are replaced by their typed values as defined in [XDM 4.0]. Arrays (see 22 Arrays) are atomized by atomizing their members, recursively.] For some items (for example, elements with element-only content, function items, and maps, atomization raises a dynamic error.

• [Definition: The term typed value is defined in [XDM 4.0] section 7.6.14 typed-value Accessor. Every node, other than an element whose type annotation identifies it as having element-only content, has a typed value. For example, the typed value of an attribute of type xs:IDREFS is a sequence of zero or more xs:IDREF values.][Definition: [Definition: The term typed value is defined in [XDM 4.0] section 7.6.14 typed-value Accessor. Every node, other than an element whose type annotation identifies it as having element-only content, has a typed value. For example, the typed value of an attribute of type xs:IDREFS is a sequence of zero or more xs:IDREF values.][Definition: [Definition: [Definition: The term typed value is defined in [XDM 4.0] section 7.6.14 typed-value Accessor. Every node, other than an element whose type annotation identifies it as having element-only content, has a typed value. For example, the typed value of an attribute of type xs:IDREFS is a sequence of zero or more xs:IDREF values.]

• [Definition: The term string value is defined in [XDM 4.0] section 7.6.12 string-value Accessor. Every node has a string value. For example, the string value of an element is the concatenation of the string values of all its descendant text nodes.][Definition: [Definition: The term string value is defined in [XDM 4.0] section 7.6.12 string-value Accessor. Every node has a string value. For example, the string value of an element is the concatenation of the string values of all its descendant text nodes.][Definition: [Definition: [Definition: The term string value is defined in [XDM 4.0] section 7.6.12 string-value Accessor. Every node has a string value. For example, the string value of an element is the concatenation of the string values of all its descendant text nodes.]

• [Definition: The term XPath 1.0 compatibility mode is defined in [XPath 4.0] section 2.2.1 Static Context. This is a setting in the static context of an XPath expression; it has two values, true and false. When the value is set to true, the semantics of function calls and certain other operations are adjusted to give a greater degree of backwards compatibility between XPath 4.0 and XPath 1.0.][Definition: [Definition: The term XPath 1.0 compatibility mode is defined in [XPath 4.0] section 2.2.1 Static Context. This is a setting in the static context of an XPath expression; it has two values, true and false. When the value is set to true, the semantics of function calls and certain other operations are adjusted to give a greater degree of backwards compatibility between XPath 4.0 and XPath 1.0.][Definition: [Definition: [Definition: The term XPath 1.0 compatibility mode is defined in [XPath 4.0] section 2.2.1 Static Context. This is a setting in the static context of an XPath expression; it has two values, true and false. When the value is set to true, the semantics of function calls and certain other operations are adjusted to give a greater degree of backwards compatibility between XPath 4.0 and XPath 1.0.]

• [Definition: The term function definition is defined in [XPath 4.0] section 2.2.1 Static Context. It is the definition of a function that can be called statically from within an XPath expression: in the case of XSLT it typically means either a stylesheet function, or a built-in function such as those defined in [Functions and Operators 4.0]][Definition: [Definition: The term function definition is defined in [XPath 4.0] section 2.2.1 Static Context. It is the definition of a function that can be called statically from within an XPath expression: in the case of XSLT it typically means either a stylesheet function, or a built-in function such as those defined in [Functions and Operators 4.0]][Definition: [Definition: [Definition: The term function definition is defined in [XPath 4.0] section 2.2.1 Static Context. It is the definition of a function that can be called statically from within an XPath expression: in the case of XSLT it typically means either a stylesheet function, or a built-in function such as those defined in [Functions and Operators 4.0]]

• [Definition: A function definition has an arity range, which defines the minimum and maximum number of arguments that must be supplied in a call to the function. The static context can contain multiple function definitions with the same name, provided that their arity ranges do not overlap.][Definition: [Definition: A function definition has an arity range, which defines the minimum and maximum number of arguments that must be supplied in a call to the function. The static context can contain multiple function definitions with the same name, provided that their arity ranges do not overlap.][Definition: [Definition: [Definition: A function definition has an arity range, which defines the minimum and maximum number of arguments that must be supplied in a call to the function. The static context can contain multiple function definitions with the same name, provided that their arity ranges do not overlap.]

2.2 Notation

[Definition: An XSLT element is an element in the XSLT namespace whose syntax and semantics are defined in this specification.] For a non-normative list of XSLT elements, see C Element Syntax Summary.[Definition: [Definition: An XSLT element is an element in the XSLT namespace whose syntax and semantics are defined in this specification.] For a non-normative list of XSLT elements, see C Element Syntax Summary.[Definition: [Definition: [Definition: An XSLT element is an element in the XSLT namespace whose syntax and semantics are defined in this specification.] For a non-normative list of XSLT elements, see C Element Syntax Summary.

In this document the specification of each XSLT element is preceded by a summary of its syntax in the form of a model for elements of that element type. A full list of all these specifications can be found in C Element Syntax Summary. The meaning of the syntax summary notation is as follows:

• An attribute that is required is shown with its name in bold. An attribute that may be omitted is shown with a question mark following its name.

• An attribute that is deprecated is shown in a grayed font within square brackets.

• The string that occurs in the place of an attribute value specifies the allowed values of the attribute. If this is surrounded by curly brackets ({...}), then the attribute value is treated as an attribute value template, and the string occurring within curly brackets specifies the allowed values of the result of evaluating the attribute value template. Alternative allowed values are separated by |. A quoted string indicates a value equal to that specific string. An unquoted, italicized name specifies a particular type of value.

  The types used, and their meanings, are as follows:

  boolean

  One of the strings "yes", "true", or "1" to indicate the value true, or one of the strings "no", "false", or "0" to indicate the value false. Note: the values are synonyms; where this specification uses a phrase such as “If required='yes' is specified ...” this is to be interpreted as meaning “If the attribute named required is present, and has the value yes, true, or 1 (after stripping leading and trailing whitespace) ...”.

  string

  Any string.

  expression

  An XPath expression.

  pattern

  A pattern as described in 6.3 Patterns.

  item-type

  An ItemType^XP as defined in the XPath 4.0 specification.

  sequence-type

  A SequenceType^XP as defined in the XPath 4.0 specification.

  uri; uris

  A URI, for example a namespace URI or a collation URI; a whitespace-separated list of URIs.

  qname

  A lexical QName as defined in 5.1.1 Qualified Names.

  eqname; eqnames

  An EQName as defined in 5.1.1 Qualified Names; a whitespace-separated list of EQNames.

  token; tokens

  A string containing no significant whitespace; a whitespace-separated list of such strings.

  nmtoken; nmtokens

  A string conforming to the XML schema rules for the type xs:NMTOKEN; a whitespace-separated list of such strings.

  char

  A string comprising a single Unicode character.

  language

  A string in the value space of xs:language, or a zero-length string.

  integer

  An integer, that is, a string that is castable to the schema type xs:integer.

  decimal

  A decimal value, that is, a string that is castable to the schema type xs:decimal.

  ncname; ncnames

  An unprefixed name: a string that is castable to the schema type xs:NCName; a whitespace-separated list of such strings.

  prefix; prefixes

  An xs:NCName representing a namespace prefix, which must be in scope for the element on which it appears; a whitespace-separated list of such strings.

  id

  An xs:NCName used as a unique identifier for an element in the containing XML document.

  Except where the set of allowed values of an attribute is specified using the italicized name string or char, leading and trailing whitespace in the attribute value is ignored. In the case of an attribute value template, this applies to the effective value obtained when the attribute value template is expanded.

  XPath comments (delimited by (: ... :)) are permitted anywhere that inter-token whitespace is permitted in attributes whose type is given as expression, pattern, item-type, or sequence-type, and are not permitted in attributes of other types (other than within expressions enclosed by curly braces within an attribute value template).

• If an attribute has a simple default value, this is shown between tortoise-shell brackets (for example 〔'no'〕). Where no default is shown, the consequence of omitting the attribute is explained in the prose narrative. Default values shown in the summary apply only where the attribute itself is applicable; if an attribute is not permitted to appear in the particular context, then its default value should be ignored. (For example, the stable attribute of xsl:sort is shown as having a default value of 'yes', but the attribute is allowed only on the first of a sequence of adjacent xsl:sort elements.) The quotation marks around a default value are not part of the value.

• Unless the element is required to be empty, the model element contains a comment specifying the allowed content. The allowed content is specified in a way similar to an element type declaration in XML; sequence constructor means that any mixture of text nodes, literal result elements, extension instructions, and XSLT elements from the instruction category is allowed; other-declarations means that any mixture of XSLT elements from the declaration category is allowed, together with user-defined data elements.

• The element is prefaced by comments indicating if it belongs to the instruction category or declaration category or both. The category of an element affects only whether it is allowed in the content of elements that allow a sequence constructor or other-declarations.

[ERR XTSE0010] It is a static error if an XSLT-defined element is used in a context where it is not permitted, if a required attribute is omitted, or if the content of the element does not correspond to the content that is allowed for the element.

The rules in the element syntax summary (both for the element structure and for its attributes) apply to the stylesheet content after preprocessing as described in 3.11 Stylesheet Preprocessing.

[Definition: The effective value of an attribute or text node in the stylesheet is the value after any required expansion or normalization.][Definition: [Definition: The effective value of an attribute or text node in the stylesheet is the value after any required expansion or normalization.][Definition: [Definition: [Definition: The effective value of an attribute or text node in the stylesheet is the value after any required expansion or normalization.]

More specifically, the effective value is the value after:

• Expanding shadow attributes as described in 3.11.4 Shadow Attributes;

• Expanding defaults (for example, if an xsl:message instruction has no terminate attribute, then the effective value of the terminate attribute is no);

• Stripping ignored whitespace (for example, the effective value of a boolean attribute written as terminate=" no " is no);

• Replacing synonyms (for example in boolean attributes, 1 and true are synonyms of yes);

• Expanding attribute value templates and text value templates.

• Applying rules from the static context: for example, the effective value of a collation attribute is the value after expanding a relative URI against the static base URI.

Attributes are validated as follows. These rules apply to the value of the attribute after removing leading and trailing whitespace.

• [ERR XTSE0020] It is a static error if an attribute (other than an attribute written using curly brackets in a position where an attribute value template is permitted) contains a value that is not one of the permitted values for that attribute.

• [ERR XTDE0030] It is a dynamic error if the effective value of an attribute written using curly brackets, in a position where an attribute value template is permitted, is a value that is not one of the permitted values for that attribute. If the processor is able to detect the error statically (for example, when any XPath expressions within the curly brackets can be evaluated statically), then the processor may optionally raise this as a static error.

Special rules apply if the construct appears in part of the stylesheet that is processed with forwards compatible behavior: see 3.9 Forwards Compatible Processing.

[Definition: Some constructs defined in this specification are described as being deprecated. The use of this term implies that stylesheet authors should not use the construct, and that the construct may be removed in a later version of this specification.][Definition: [Definition: Some constructs defined in this specification are described as being deprecated. The use of this term implies that stylesheet authors should not use the construct, and that the construct may be removed in a later version of this specification.][Definition: [Definition: [Definition: Some constructs defined in this specification are described as being deprecated. The use of this term implies that stylesheet authors should not use the construct, and that the construct may be removed in a later version of this specification.]

XSLT defines a set of standard functions which are additional to those defined in [Functions and Operators 4.0]. A list of these functions appears in G.2 List of XSLT-defined functions. The signatures of these functions are described using the same notation as used in [Functions and Operators 4.0]. The names of many of these functions are in the standard function namespace.

2.3 Initiating a Transformation

This document does not specify any application programming interfaces or other interfaces for initiating a transformation. This section, however, describes the information that is supplied when a transformation is initiated. Except where otherwise indicated, the information is required.

The execution of a stylesheet necessarily involves two activities: static analysis and dynamic evaluation. Static analysis consists of those tasks that can be performed by inspection of the stylesheet alone, including the binding of static variables, the evaluation of [xsl:]use-when expressions (see 3.11.3 Conditional Element Inclusion) and shadow attributes (see 3.11.4 Shadow Attributes), and the detection of static errors. Dynamic evaluation consists of tasks which in general cannot be carried out until a source document is available.

Dynamic evaluation is further divided into two activities: priming the stylesheet, and invoking a selected component.

• Priming the stylesheet provides the dynamic context for evaluation, and supplies all the information needed to establish the values of global variables.

• Invoking a component (such as a template or function) causes evaluation of that template or function to produce a result, which is an arbitrary XDM value.

  [Definition: The result of invoking the selected component, after any required conversion to the declared result type of the component, is referred to as the raw result.][Definition: [Definition: The result of invoking the selected component, after any required conversion to the declared result type of the component, is referred to as the raw result.][Definition: [Definition: [Definition: The result of invoking the selected component, after any required conversion to the declared result type of the component, is referred to as the raw result.]

  The raw result of the invocation is the immediate result of evaluating the sequence constructor contained in the target template or function, modified by applying the coercion rules to convert the immediate result to the type declared in the as attribute of the xsl:template or xsl:function declaration, if present.

  This raw result may optionally be post-processed to construct a result tree, to serialize the result, or both, as described in 2.3.6 Post-processing the Raw Result.

Implementations may allow static analysis and dynamic evaluation to be initiated independently, so that the cost of static analysis can be amortized over multiple transformations using the same stylesheet. Implementations may also allow priming of a stylesheet and invocation of components to be initiated independently, in which case a single act of priming the stylesheet may be followed by a series of independent component invocations. Although this specification does not require such a separation, this section distinguishes information that is needed before static analysis can proceed, information that is needed to prime the stylesheet, and information that is needed when invoking components.

The language is designed to allow the static analysis of each package to be performed independently of other packages, with only basic knowledge of the properties of components made available by used packages. Beyond this, the specification leaves it to implementations to decide how to organize this process. When packages are not used explicitly, the entire stylesheet is treated as a single package.

2.4 Instructions

The main executable components of a stylesheet are templates and functions. The body of a template or function is a sequence constructor, which is a sequence of elements and text nodes that can be evaluated to produce a result.

A sequence constructor is a sequence of sibling nodes in the stylesheet, each of which is either an XSLT instruction, a literal result element, a text node, or an extension instruction.

[Definition: An instruction is either an XSLT instruction or an extension instruction.][Definition: [Definition: An instruction is either an XSLT instruction or an extension instruction.][Definition: [Definition: [Definition: An instruction is either an XSLT instruction or an extension instruction.]

[Definition: An XSLT instruction is an XSLT element whose syntax summary in this specification contains the annotation <!-- category: instruction -->.][Definition: [Definition: An XSLT instruction is an XSLT element whose syntax summary in this specification contains the annotation <!-- category: instruction -->.][Definition: [Definition: [Definition: An XSLT instruction is an XSLT element whose syntax summary in this specification contains the annotation <!-- category: instruction -->.]

Extension instructions are described in 24.3 Extension Instructions.

The main categories of XSLT instruction are as follows:

• instructions that create new nodes: xsl:document, xsl:element, xsl:attribute, xsl:processing-instruction, xsl:comment, xsl:value-of, xsl:text, xsl:namespace;

• instructions that construct maps and arrays: xsl:array, xsl:map, xsl:map-entry, xsl:record;

• instructions that copy nodes: xsl:copy, xsl:copy-of;

• instructions that returns an arbitrary sequence by evaluating an XPath expression: xsl:sequence, xsl:select, xsl:evaluate;

• instructions that cause conditional or repeated evaluation of nested instructions: xsl:if, xsl:choose, xsl:switch, xsl:try, xsl:for-each, xsl:for-each-group, xsl:fork, xsl:iterate and its subordinate instructions xsl:next-iteration and xsl:break;

• instructions that generate output conditionally if elements are or are not empty: xsl:on-empty, xsl:on-non-empty, xsl:where-populated;

• instructions that invoke templates: xsl:apply-templates, xsl:apply-imports, xsl:call-template, xsl:next-match;

• Instructions that declare variables: xsl:variable;

• Instructions to assist debugging: xsl:message, xsl:assert;

• other specialized instructions: xsl:number, xsl:analyze-string, xsl:fork, xsl:result-document, xsl:source-document, xsl:perform-sort, xsl:merge.

2.5 Rule-Based Processing

The classic method of executing an XSLT transformation is to apply template rules to the root node of an input document (see 2.3.3 Apply-Templates Invocation). The operation of applying templates to a node searches the stylesheet for the best matching template rule for that node. This template rule is then evaluated. A common coding pattern, especially when XSLT is used to convert XML documents into display formats such as HTML, is to have one template rule for each kind of element in the source document, and for that template rule to generate some appropriate markup elements, and to apply templates recursively to its own children. The effect is to perform a recursive traversal of the source tree, in which each node is processed using the best-fit template rule for that node. The final result of the transformation is then the tree produced by this recursive process. This result can then be optionally serialized (see 2.3.6 Post-processing the Raw Result).

  <PERSONAE PLAY="OTHELLO">
    <TITLE>Dramatis Personae</TITLE>
    <PERSONA>DUKE OF VENICE</PERSONA>
    <PERSONA>BRABANTIO, a senator.</PERSONA>
    <PERSONA>Other Senators.</PERSONA>
    <PERSONA>GRATIANO, brother to Brabantio.</PERSONA>
    <PERSONA>LODOVICO, kinsman to Brabantio.</PERSONA>
    <PERSONA>OTHELLO, a noble Moor in the service of the Venetian state.</PERSONA>
    <PERSONA>CASSIO, his lieutenant.</PERSONA>
    <PERSONA>IAGO, his ancient.</PERSONA>
    <PERSONA>RODERIGO, a Venetian gentleman.</PERSONA>
    <PERSONA>MONTANO, Othello's predecessor in the government of Cyprus.</PERSONA>
    <PERSONA>Clown, servant to Othello. </PERSONA>
    <PERSONA>DESDEMONA, daughter to Brabantio and wife to Othello.</PERSONA>
    <PERSONA>EMILIA, wife to Iago.</PERSONA>
    <PERSONA>BIANCA, mistress to Cassio.</PERSONA>
    <PERSONA>Sailor, Messenger, Herald, Officers, 
             Gentlemen, Musicians, and Attendants.</PERSONA>
  </PERSONAE>

 <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    version="3.0"
    expand-text="yes">
    
 <xsl:strip-space elements="PERSONAE"/>   
    
 <xsl:template match="PERSONAE">
   <html>
     <head>
       <title>The Cast of {@PLAY}</title>
     </head>
     <body>
       <xsl:apply-templates/>
     </body>
   </html>
 </xsl:template>
 
 <xsl:template match="TITLE">
   <h1>{.}</h1>
 </xsl:template>
 
 <xsl:template match="PERSONA[count(tokenize(., ',')) = 2]">
   <p><b>{substring-before(., ',')}</b>: {substring-after(., ',')}</p>
 </xsl:template> 

 <xsl:template match="PERSONA">
   <p><b>{.}</b></p>
 </xsl:template>

</xsl:stylesheet>

[Definition: A stylesheet contains a set of template rules (see 6 Template Rules). A template rule has three parts: a pattern that is matched against selected items (often but not necessarily nodes), a (possibly empty) set of template parameters, and a sequence constructor that is evaluated to produce a sequence of items.] In many cases these items are newly constructed nodes, which are then written to a result tree.[Definition: [Definition: A stylesheet contains a set of template rules (see 6 Template Rules). A template rule has three parts: a pattern that is matched against selected items (often but not necessarily nodes), a (possibly empty) set of template parameters, and a sequence constructor that is evaluated to produce a sequence of items.] In many cases these items are newly constructed nodes, which are then written to a result tree.[Definition: [Definition: [Definition: A stylesheet contains a set of template rules (see 6 Template Rules). A template rule has three parts: a pattern that is matched against selected items (often but not necessarily nodes), a (possibly empty) set of template parameters, and a sequence constructor that is evaluated to produce a sequence of items.] In many cases these items are newly constructed nodes, which are then written to a result tree.

2.7 Parsing and Serialization

An XSLT stylesheet describes a process that constructs a set of results from a set of inputs. The inputs are the data provided at stylesheet invocation, as described in 2.3 Initiating a Transformation. The results include the principal result (an arbitrary sequence), which is the result of the initial component invocation, together with any secondary results produced using xsl:result-document instructions.

The stylesheet does not describe how a source tree is constructed. Some possible ways of constructing source trees are described in [XDM 4.0]. Frequently an implementation will operate in conjunction with an XML parser (or more strictly, in the terminology of [XML 1.0], an XML processor), to build a source tree from an input XML document. An implementation may also provide an application programming interface allowing the tree to be constructed directly, or allowing it to be supplied in the form of a DOM Document object (see [DOM Level 2]). This is outside the scope of this specification. Users should be aware, however, that since the input to the transformation is a tree conforming to the XDM data model as described in [XDM 4.0], constructs that might exist in the original XML document, or in the DOM, but which are not within the scope of the data model, cannot be processed by the stylesheet and cannot be guaranteed to remain unchanged in the transformation output. Such constructs include CDATA section boundaries, the use of entity references, and the DOCTYPE declaration and internal DTD subset.

[Definition: A frequent requirement is to output a final result tree as an XML document (or in other formats such as HTML). This process is referred to as serialization.][Definition: [Definition: A frequent requirement is to output a final result tree as an XML document (or in other formats such as HTML). This process is referred to as serialization.][Definition: [Definition: [Definition: A frequent requirement is to output a final result tree as an XML document (or in other formats such as HTML). This process is referred to as serialization.]

Like parsing, serialization is not part of the transformation process, and it is not required that an XSLT processor must be able to perform serialization. However, for pragmatic reasons, this specification describes declarations (the xsl:output element and the xsl:character-map declarations, see 26 Serialization), and attributes on the xsl:result-document instruction, that allow a stylesheet to specify the desired properties of a serialized output file. When serialization is not being performed, either because the implementation does not support the serialization option, or because the user is executing the transformation in a way that does not invoke serialization, then the content of the xsl:output and xsl:character-map declarations has no effect. Under these circumstances the processor may raise any errors in an xsl:output or xsl:character-map declaration, or in the serialization attributes of xsl:result-document, but is not required to do so.

2.10 Stylesheets and XML Schemas

An XSLT stylesheet can make use of information from a schema. An XSLT transformation can take place in the absence of a schema (and, indeed, in the absence of a DTD), but where the source document has undergone schema validity assessment, the XSLT processor has access to the type information associated with individual nodes, not merely to the untyped text.

Information from a schema can be used both statically (when the stylesheet is compiled), and dynamically (during evaluation of the stylesheet to transform a source document).

There are places within a stylesheet, and within XPath expressions and patterns in a stylesheet, where it is possible to refer to named type definitions in a schema, or to element and attribute declarations. For example, it is possible to declare the types expected for the parameters of a function. This is done using a SequenceType.

[Definition: A SequenceType constrains the type and number of items in a sequence. The term is used both to denote the concept, and to refer to the syntactic form in which sequence types are expressed in the XPath grammar: specifically SequenceType^XP in [XPath 4.0].][Definition: [Definition: A SequenceType constrains the type and number of items in a sequence. The term is used both to denote the concept, and to refer to the syntactic form in which sequence types are expressed in the XPath grammar: specifically SequenceType^XP in [XPath 4.0].][Definition: [Definition: [Definition: A SequenceType constrains the type and number of items in a sequence. The term is used both to denote the concept, and to refer to the syntactic form in which sequence types are expressed in the XPath grammar: specifically SequenceType^XP in [XPath 4.0].]

[Definition: Type definitions and element and attribute declarations are referred to collectively as schema components.][Definition: [Definition: Type definitions and element and attribute declarations are referred to collectively as schema components.][Definition: [Definition: [Definition: Type definitions and element and attribute declarations are referred to collectively as schema components.]

[Definition: The schema components that may be referenced by name in a package are referred to as the in-scope schema components.][Definition: [Definition: The schema components that may be referenced by name in a package are referred to as the in-scope schema components.][Definition: [Definition: [Definition: The schema components that may be referenced by name in a package are referred to as the in-scope schema components.]

The set of in-scope schema components may vary between one package and another, and between different parts of the same package, but as explained in 3.13 Importing Schema Components, the schema components used in different parts of the stylesheet must be consistent with each other.

The conformance rules for XSLT 4.0, defined in 27 Conformance, distinguish between a basic XSLT processor and a schema-aware XSLT processor. As the names suggest, a basic XSLT processor does not support the features of XSLT that require access to schema information, either statically or dynamically. A stylesheet that works with a basic XSLT processor will produce the same results with a schema-aware XSLT processor provided that the source documents are untyped (that is, they are not validated against a schema). However, if source documents are validated against a schema then the results may be different from the case where they are not validated. Some constructs that work on untyped data may fail with typed data (for example, an attribute of type xs:date cannot be used as an argument of the substring function) and other constructs may produce different results depending on the datatype (for example, given the element <product price="10.00" discount="2.00"/>, the expression @price gt @discount will return true if the attributes have type xs:decimal, but will return false if they are untyped).

There is a standard set of type definitions that are always available as in-scope schema components in every stylesheet. These are defined in 3.12 Built-in Types.

The remainder of this section describes facilities that are available only with a schema-aware XSLT processor.

Additional schema components (type definitions, element declarations, and attribute declarations) may be added to the in-scope schema components by means of the xsl:import-schema declaration in a stylesheet.

The xsl:import-schema declaration may reference an external schema document by means of a URI, or it may contain an inline xs:schema element.

An xsl:import-schema declaration may include a role attribute, which indicates that the schema components are only to be present in the static context of a region of the stylesheet where they are explicitly invoked by means of an [xsl:]schema-role attribute.

It is only necessary to import a schema explicitly if one or more of its schema components are referenced explicitly by name in the stylesheet; it is not necessary to import a schema merely because the stylesheet is used to process a source document that has been assessed against that schema. It is possible to make use of the information resulting from schema assessment (for example, the fact that a particular attribute holds a date) even if no schema has been imported by the stylesheet.

Importing a schema does not of itself say anything about the type of the source document that the stylesheet is expected to process. The imported type definitions can be used for temporary nodes or for nodes on a result tree just as much as for nodes in source documents. It is possible to make assertions about the type of an input document by means of tests within the stylesheet. For example:

<xsl:mode typed="lax"/>
<xsl:global-context-item use="required"
            as="document-node(schema-element(my:invoice))"/>

It is also true that importing a schema does not of itself say anything about the structure of the result tree. It is possible to request validation of a result tree against the schema by using the xsl:result-document instruction, for example:

<xsl:template match="/">
  <xsl:result-document validation="strict">
    <xhtml:html>
      <xsl:apply-templates/>
    </xhtml:html>
  </xsl:result-document>
</xsl:template>

It is possible that a source document may contain nodes whose type annotation is not one of the types imported by the stylesheet. The data model (see [XDM 4.0]) requires that the processor has sufficient information about such types to correctly implement the semantics of the language, even though the schema has not been imported; it also requires that all schemas used by the processor in a given processing episode (for example, an XSLT transformation) should be consistent. For example, if a type annotation of a node is the simple type my:percentage, and a type named my:percentage appears in an imported schema, then they must be the same type.

Where a stylesheet author chooses to make assertions about the types of nodes or of variables and parameters, it is possible for an XSLT processor to perform static analysis of the stylesheet (that is, analysis in the absence of any source document). Such analysis may reveal errors that would otherwise not be discovered until the transformation is actually executed. An XSLT processor is not required to perform such static type-checking. Under some circumstances (see 2.12 Error Handling) type errors that are detected early may be raised as static errors. In addition an implementation may report any condition found during static analysis as a warning, provided that this does not prevent the stylesheet being evaluated as described by this specification.

A stylesheet can also control the type annotations of nodes that it constructs in a result tree. This can be done in a number of ways.

• It is possible to request explicit validation of a complete document, that is, a result tree rooted at a document node. Validation is either strict or lax, as described in [XML Schema Part 1]. If validation of a result tree fails (strictly speaking, if the outcome of the validity assessment is invalid), then the transformation fails, but in all other cases, the element and attribute nodes of the tree will be annotated with the names of the types to which these nodes conform. These type annotations will be discarded if the result tree is serialized as an XML document, but they remain available when the result tree is passed to an application (perhaps another stylesheet) for further processing.

• It is also possible to validate individual element and attribute nodes as they are constructed. This is done using the type and validation attributes of the xsl:element, xsl:attribute, xsl:copy, and xsl:copy-of instructions, or the xsl:type and xsl:validation attributes of a literal result element.

• When elements, attributes, or document nodes are copied, either explicitly using the xsl:copy or xsl:copy-of instructions, or implicitly when nodes in a sequence are attached to a new parent node, the options validation="strip" and validation="preserve" are available, to control whether existing type annotations are to be retained or not.

When nodes in a temporary tree are validated, type information is available for use by operations carried out on the temporary tree, in the same way as for a source document that has undergone schema assessment.

For details of how validation of element and attribute nodes works, see 25.4 Validation.

2.11 Streaming

Streaming is an optional feature of the XSLT 4.0 language.

[Definition: The term streaming refers to a manner of processing in which XML documents (such as source and result documents) are not represented by a complete tree of nodes occupying memory proportional to document size, but instead are processed “on the fly” as a sequence of events, similar in concept to the stream of events notified by an XML parser to represent markup in lexical XML.][Definition: [Definition: The term streaming refers to a manner of processing in which XML documents (such as source and result documents) are not represented by a complete tree of nodes occupying memory proportional to document size, but instead are processed “on the fly” as a sequence of events, similar in concept to the stream of events notified by an XML parser to represent markup in lexical XML.][Definition: [Definition: [Definition: The term streaming refers to a manner of processing in which XML documents (such as source and result documents) are not represented by a complete tree of nodes occupying memory proportional to document size, but instead are processed “on the fly” as a sequence of events, similar in concept to the stream of events notified by an XML parser to represent markup in lexical XML.]

[Definition: A streamed document is a source tree that is processed using streaming, that is, without constructing a complete tree of nodes in memory.][Definition: [Definition: A streamed document is a source tree that is processed using streaming, that is, without constructing a complete tree of nodes in memory.][Definition: [Definition: [Definition: A streamed document is a source tree that is processed using streaming, that is, without constructing a complete tree of nodes in memory.]

[Definition: A streamed node is a node in a streamed document.][Definition: [Definition: A streamed node is a node in a streamed document.][Definition: [Definition: [Definition: A streamed node is a node in a streamed document.]

For full information about streaming, see [XSLT 4.0 Streaming].

2.12 Error Handling

[Definition: An error that can be detected by examining a stylesheet before execution starts (that is, before the source document and values of stylesheet parameters are available) is referred to as a static error.][Definition: [Definition: An error that can be detected by examining a stylesheet before execution starts (that is, before the source document and values of stylesheet parameters are available) is referred to as a static error.][Definition: [Definition: [Definition: An error that can be detected by examining a stylesheet before execution starts (that is, before the source document and values of stylesheet parameters are available) is referred to as a static error.]

Generally, errors in the structure of the stylesheet, or in the syntax of XPath expressions contained in the stylesheet, are classified as static errors. Where this specification states that an element in the stylesheet must or must not appear in a certain position, or that it must or must not have a particular attribute, or that an attribute must or must not have a value satisfying specified conditions, then any contravention of this rule is a static error unless otherwise specified.

A processor must provide a mode of operation that takes a (possibly erroneous) stylesheet package as input and enables the user to determine whether or not that package contains any static errors.

A processor may also provide a mode of operation in which static errors in parts of the stylesheet that are not evaluated can go unreported.

[Definition: An error that is not capable of detection until a source document is being transformed is referred to as a dynamic error.][Definition: [Definition: An error that is not capable of detection until a source document is being transformed is referred to as a dynamic error.][Definition: [Definition: [Definition: An error that is not capable of detection until a source document is being transformed is referred to as a dynamic error.]

When a dynamic error occurs, and is not caught using xsl:catch, the processormust raise the error, and the transformation fails.

Because different implementations may optimize execution of the stylesheet in different ways, the detection of dynamic errors is to some degree implementation-dependent. In cases where an implementation is able to produce a principal result or secondary result without evaluating a particular construct, the implementation is never required to evaluate that construct solely in order to determine whether doing so causes a dynamic error. For example, if a variable is declared but never referenced, an implementation may choose whether or not to evaluate the variable declaration, which means that if evaluating the variable declaration causes a dynamic error, some implementations will raise this error and others will not.

There are some cases where this specification requires that a construct must not be evaluated: for example, the content of an xsl:if instruction must not be evaluated if the test condition is false. This means that an implementation must not raise any dynamic errors that would arise if the construct were evaluated.

An implementation may raise a dynamic error before any source document is available, but only if it can determine that the error would be raised for every possible source document and every possible set of parameter values. For example, some circularity errors fall into this category: see 9.11 Circular Definitions.

There are also some dynamic errors where the specification gives a processor license to raise the error during the analysis phase even if the construct might never be executed; an example is the use of an invalid QName as a literal argument to a function such as key, or the use of an invalid regular expression in the regex attribute of the xsl:analyze-string instruction.

A dynamic error is also raised during the static analysis phase if the error occurs during evaluation of a static expression.

The XPath specification states (see [XPath 4.0] section 2.5.1 Kinds of Errors) that if any expression (at any level) can be evaluated during the analysis phase (because all its explicit operands are known and it has no dependencies on the dynamic context), then any error in performing this evaluation may be raised as a static error. For XPath expressions used in an XSLT stylesheet, however, any such errors must not be raised as static errors in the stylesheet unless they would occur in every possible evaluation of that stylesheet; instead, they must be raised as dynamic errors, and raised only if the XPath expression is actually evaluated.

<xsl:choose>
  <xsl:when test="system-property('xsl:version') = '1.0'">
    <xsl:value-of select="1 div 0"/>
  </xsl:when>
  <xsl:otherwise>
    <xsl:value-of select="xs:double('INF')"/>
  </xsl:otherwise>
</xsl:choose>

[Definition: Certain errors are classified as type errors. A type error occurs when the value supplied as input to an operation is of the wrong type for that operation, for example when an integer is supplied to an operation that expects a node.] If a type error occurs in an instruction that is actually evaluated, then it must be raised in the same way as a dynamic error. Alternatively, an implementation may raise a type error during the analysis phase in the same way as a static error, even if it occurs in part of the stylesheet that is never evaluated, provided it can establish that execution of a particular construct would never succeed.[Definition: [Definition: Certain errors are classified as type errors. A type error occurs when the value supplied as input to an operation is of the wrong type for that operation, for example when an integer is supplied to an operation that expects a node.] If a type error occurs in an instruction that is actually evaluated, then it must be raised in the same way as a dynamic error. Alternatively, an implementation may raise a type error during the analysis phase in the same way as a static error, even if it occurs in part of the stylesheet that is never evaluated, provided it can establish that execution of a particular construct would never succeed.[Definition: [Definition: [Definition: Certain errors are classified as type errors. A type error occurs when the value supplied as input to an operation is of the wrong type for that operation, for example when an integer is supplied to an operation that expects a node.] If a type error occurs in an instruction that is actually evaluated, then it must be raised in the same way as a dynamic error. Alternatively, an implementation may raise a type error during the analysis phase in the same way as a static error, even if it occurs in part of the stylesheet that is never evaluated, provided it can establish that execution of a particular construct would never succeed.

It is implementation-defined whether type errors are raised statically.

<xsl:if test="false()">
  <xsl:number select="42"/>
</xsl:if>

<xsl:template match="para">
  <xsl:param name="p" as="item()"/>
  <xsl:number select="$p"/>
</xsl:template>

If more than one error arises, an implementation is not required to raise any errors other than the first one that it detects. It is implementation-dependent which of the several errors is raised. This applies both to static errors and to dynamic errors. An implementation is allowed to raise more than one error, but if any errors have been raised, it must not finish as if the transformation were successful.

When a transformation raises one or more dynamic errors, the final state of any persistent resources updated by the transformation is implementation-dependent. Implementations are not required to restore such resources to their initial state. In particular, where a transformation produces multiple result documents, it is possible that one or more serialized result documents may be written successfully before the transformation terminates, but the application cannot rely on this behavior.

Everything said above about error handling applies equally to errors in evaluating XSLT instructions, and errors in evaluating XPath expressions. Static errors and dynamic errors may occur in both cases.

[Definition: If a transformation has successfully produced a principal result or secondary result, it is still possible that errors may occur in serializing that result . For example, it may be impossible to serialize the result using the encoding selected by the user. Such an error is referred to as a serialization error.] If the processor performs serialization, then it must do so as specified in 26 Serialization, and in particular it must raise any serialization errors that occur.[Definition: [Definition: If a transformation has successfully produced a principal result or secondary result, it is still possible that errors may occur in serializing that result . For example, it may be impossible to serialize the result using the encoding selected by the user. Such an error is referred to as a serialization error.] If the processor performs serialization, then it must do so as specified in 26 Serialization, and in particular it must raise any serialization errors that occur.[Definition: [Definition: [Definition: If a transformation has successfully produced a principal result or secondary result, it is still possible that errors may occur in serializing that result . For example, it may be impossible to serialize the result using the encoding selected by the user. Such an error is referred to as a serialization error.] If the processor performs serialization, then it must do so as specified in 26 Serialization, and in particular it must raise any serialization errors that occur.

Errors are identified by a QName. For errors defined in this specification, the namespace of the QName is always http://www.w3.org/2005/xqt-errors (and is therefore not given explicitly), while the local part is an 8-character code in the form PPSSNNNN. Here PP is always XT (meaning XSLT), and SS is one of SE (static error), DE (dynamic error), or TE (type error). Note that the allocation of an error to one of these categories is purely for convenience and carries no normative implications about the way the error is handled. Many errors, for example, can be raised either dynamically or statically. These error codes are used to label error conditions in this specification, and are summarized in D Summary of Error Conditions.

Errors defined in related specifications ([XPath 4.0], [Functions and Operators 4.0][Serialization 4.0]) use QNames with a similar structure, in the same namespace. When errors occur in processing XPath expressions, an XSLT processor should use the original error code reported by the XPath processor, unless a more specific XSLT error code is available.

Implementations must use the codes defined in these specifications when raising dynamic errors, to ensure that xsl:catch behaves in an interoperable way across implementations. Stylesheet authors should note, however, that there are many examples of errors where more than one rule in this specification is violated, and where the processor therefore has discretion in deciding which error code to associate with the condition: there is therefore no guarantee that different processors will always use the same error code for the same erroneous input.

Additional errors defined by an implementation (or by an application) may use QNames in an implementation-defined (or user-defined) namespace without risk of collision.

3.1 Namespaces

A stylesheet module is typically written as a namespace well-formed XML document: more formally, as described in 3.5 Stylesheet Modules, a stylesheet module takes the form of an XDM tree rooted at an element node.

The names of elements and attributes in source documents and result documents are namespace-qualified names. In addition, as described in 5.1.1 Qualified Names, XSLT uses namespace-qualified names to identify variables, functions, templates, and other components. These names generally use namespace prefixes that are resolved to namespace URIs using a set of prefix-uri namespace bindings.

There are two ways namespace bindings can be established in a stylesheet module:

• [Definition:  The native namespace bindings for any element in an XSLT stylesheet module are the prefix-uri mappings defined by the namespace nodes of that element, according to the rules in [XDM 4.0].][Definition: [Definition:  The native namespace bindings for any element in an XSLT stylesheet module are the prefix-uri mappings defined by the namespace nodes of that element, according to the rules in [XDM 4.0].][Definition: [Definition: [Definition:  The native namespace bindings for any element in an XSLT stylesheet module are the prefix-uri mappings defined by the namespace nodes of that element, according to the rules in [XDM 4.0].]

  For example, a namespace declaration of the form xmlns:math="http://www.w3.org/2005/xpath-functions/math establishes a binding of the prefix math to the namespace URI http://www.w3.org/2005/xpath-functions/math, thereby enabling functions in that namespace to be invoked using an expression such as math:sin($theta)

• [Definition:  The fixed namespace bindings for a stylesheet module are established using the fixed-namespaces attribute on the xsl:stylesheet, xsl:transform, or xsl:package element enclosing the stylesheet module.][Definition: [Definition:  The fixed namespace bindings for a stylesheet module are established using the fixed-namespaces attribute on the xsl:stylesheet, xsl:transform, or xsl:package element enclosing the stylesheet module.][Definition: [Definition: [Definition:  The fixed namespace bindings for a stylesheet module are established using the fixed-namespaces attribute on the xsl:stylesheet, xsl:transform, or xsl:package element enclosing the stylesheet module.]

  For example, the attribute fixed-namespaces="math map array" establishes bindings for the prefixes math, map, and array to the namespace URIs conventionally associated with these prefixes as described in 5.1.3 Reserved Namespaces.

Prefixes used in element and attribute names in the stylesheet, because these are interpreted by the XML parser and not only by the XSLT processor, must be bound using native namespace bindings. In particular, the XSLT namespace is used in the names of XSLT elements, so it must be declared in every stylesheet module using a namespace declaration such as xmlns:xsl="http://www.w3.org/1999/XSL/Transform. (A different prefix can be used: some users prefer xslt, some favor the default namespace.) But namespace prefixes that are only used within the content of attribute and text nodes in the stylesheet (for example, select="math:sin($theta)") can be declared in fixed namespace bindings.

[Definition:  The applicable static namespaces for an element in a stylesheet module are the fixed namespace bindings for the module if the root element of the module has a fixed-namespaces attribute, or the native namespace bindings of the element otherwise. ][Definition: [Definition:  The applicable static namespaces for an element in a stylesheet module are the fixed namespace bindings for the module if the root element of the module has a fixed-namespaces attribute, or the native namespace bindings of the element otherwise. ][Definition: [Definition: [Definition:  The applicable static namespaces for an element in a stylesheet module are the fixed namespace bindings for the module if the root element of the module has a fixed-namespaces attribute, or the native namespace bindings of the element otherwise. ]

The effect of declaring fixed namespace bindings is described in more detail in 3.6.1 The fixed-namespaces Attribute.

3.3 Standard Attributes

[Definition: There are a number of standard attributes that may appear on any XSLT element: specifically default-collation, default-mode, default-validation, exclude-result-prefixes, expand-text, extension-element-prefixes, schema-role, use-when, version, and xpath-default-namespace.][Definition: [Definition: There are a number of standard attributes that may appear on any XSLT element: specifically default-collation, default-mode, default-validation, exclude-result-prefixes, expand-text, extension-element-prefixes, schema-role, use-when, version, and xpath-default-namespace.][Definition: [Definition: [Definition: There are a number of standard attributes that may appear on any XSLT element: specifically default-collation, default-mode, default-validation, exclude-result-prefixes, expand-text, extension-element-prefixes, schema-role, use-when, version, and xpath-default-namespace.]

These attributes may also appear on a literal result element, but in this case, to distinguish them from user-defined attributes, the names of the attributes are in the XSLT namespace. They are thus typically written as xsl:default-collation, xsl:default-mode, xsl:default-validation, xsl:exclude-result-prefixes, xsl:expand-text, xsl:extension-element-prefixes, xsl:use-when, xsl:version, or xsl:xpath-default-namespace.

It is recommended that all these attributes should also be permitted on extension instructions, but this is at the discretion of the implementer of each extension instruction. They may also be permitted on user-defined data elements, though they will only have any useful effect in the case of data elements that are designed to behave like XSLT declarations or instructions.

In the following descriptions, these attributes are referred to generically as [xsl:]version, and so on.

These attributes all affect the element they appear on, together with any elements and attributes that have that element as an ancestor. The two forms with and without the XSLT namespace have the same effect; the XSLT namespace is used for the attribute if and only if its parent element is not in the XSLT namespace.

In the case of [xsl:]default-collation, [xsl:]expand-text, [xsl:]schema-role, [xsl:]version, and [xsl:]xpath-default-namespace, the value can be overridden by a different value for the same attribute appearing on a descendant element. The effective value of the attribute for a particular stylesheet element is determined by the innermost ancestor-or-self element on which the attribute appears.

In an embedded stylesheet module, standard attributes appearing on ancestors of the outermost element of the stylesheet module have no effect.

In the case of [xsl:]exclude-result-prefixes and [xsl:]extension-element-prefixes the values are cumulative. For these attributes, the value is given as a whitespace-separated list of namespace prefixes, and the effective value for an element is the combined set of namespace URIs designated by the prefixes that appear in this attribute for that element and any of its ancestor elements. Again, the two forms with and without the XSLT namespace are equivalent.

The effect of the [xsl:]use-when attribute is described in 3.11.3 Conditional Element Inclusion.

Because these attributes may appear on any XSLT element, they are not listed in the syntax summary of each individual element. Instead they are listed and described in the entry for the xsl:stylesheet, xsl:transform, and xsl:package elements only. This reflects the fact that these attributes are often used on the outermost element of the stylesheet, in which case they apply to the entire stylesheet module or package manifest.

Note that the effect of these attributes does not extend to stylesheet modules referenced by xsl:include or xsl:import declarations, nor to packages referenced using xsl:use-package.

For the detailed effect of each attribute, see the following sections:

[xsl:]default-collation

see 3.6.2 The default-collation Attribute

[xsl:]default-mode

see 3.6.3 The default-mode Attribute

[xsl:]default-validation

see 25.4 Validation

[xsl:]exclude-result-prefixes

see 11.1.3 Namespace Nodes for Literal Result Elements

[xsl:]expand-text

see 5.6.2 Text Value Templates

[xsl:]extension-element-prefixes

see 24.3 Extension Instructions

[xsl:]schema-role

see 3.13.1 Multiple Schemas

[xsl:]use-when

see 3.11.3 Conditional Element Inclusion

[xsl:]version

see 3.8 Backwards Compatible Processing and 3.9 Forwards Compatible Processing

[xsl:]xpath-default-namespace

see 5.1.2 Unprefixed Lexical QNames in Expressions and Patterns

3.4 Packages

[Definition: An explicit package is represented by an xsl:package element, which will generally be the outermost element of an XML document. When the xsl:package element is not used explicitly, the entire stylesheet comprises a single implicit package.] (This specification does not preclude the xsl:package being embedded in another XML document, but it will never have any other XSLT element as an ancestor).[Definition: [Definition: An explicit package is represented by an xsl:package element, which will generally be the outermost element of an XML document. When the xsl:package element is not used explicitly, the entire stylesheet comprises a single implicit package.] (This specification does not preclude the xsl:package being embedded in another XML document, but it will never have any other XSLT element as an ancestor).[Definition: [Definition: [Definition: An explicit package is represented by an xsl:package element, which will generally be the outermost element of an XML document. When the xsl:package element is not used explicitly, the entire stylesheet comprises a single implicit package.] (This specification does not preclude the xsl:package being embedded in another XML document, but it will never have any other XSLT element as an ancestor).

<xsl:package
  id? = id
  name? = uri
  package-version? = string〔'1'〕
  version = decimal
  input-type-annotations? = "preserve" | "strip" | "unspecified"〔'unspecified'〕
  declared-modes? = boolean〔'yes'〕
  default-mode? = eqname | "#unnamed"〔'#unnamed'〕
  default-validation? = "preserve" | "strip"〔'strip'〕
  default-collation? = uris
  extension-element-prefixes? = prefixes
  exclude-result-prefixes? = prefixes
  expand-text? = boolean〔'no'〕
  fixed-namespaces? = string
  schema-role? = ncname
  use-when? = expression〔true()〕
  xpath-default-namespace? = uri >
  <!-- Content: ((xsl:expose | declarations)*) -->
</xsl:package>

[Definition: The content of the xsl:package element is referred to as the package manifest].[Definition: [Definition: The content of the xsl:package element is referred to as the package manifest].[Definition: [Definition: [Definition: The content of the xsl:package element is referred to as the package manifest].

The version attribute indicates the version of the XSLT language specification to which the package manifest conforms. The value should normally be 4.0. If the value is numerically less than 4.0, the content of the xsl:package element is processed using the rules for backwards compatible behavior (see 3.8 Backwards Compatible Processing). If the value is numerically greater than 4.0, it is processed using the rules for forwards compatible behavior (see 3.9 Forwards Compatible Processing).

A package typically has a name, given in its name attribute, which must be an absolute URI. Unnamed packages are allowed, but they can only be used as the “top level” of an application; they cannot be the target of an xsl:use-package declaration in another package.

A package may have a version identifier, given in its package-version attribute. This is used to distinguish different versions of a package. The value of the version attribute, after trimming leading and trailing whitespace, must conform to the syntax given in 3.4.1 Versions of a Package. If no version number is specified for a package, version 1 is assumed.

The attributes default-collation, default-mode, default-validation, exclude-result-prefixes, expand-text, extension-element-prefixes, use-when, version, and xpath-default-namespace are standard attributes that can appear on any XSLT element, and potentially affect all descendant elements. Their meaning is described in 3.3 Standard Attributes.

The package manifest contains the following elements, arbitrarily ordered:

1. Zero or more xsl:expose declarations that define the interface offered by this package to the outside world. An xsl:expose declaration may appear only as a child of xsl:package.

2. Zero or more additional declarations. These are the same as the declarations permitted as children of xsl:stylesheet or xsl:transform.

   Some declarations of particular relevance to packages include:

   1. The xsl:use-package declaration, which declares the names and versions of the packages on which this package depends.

   2. The optional xsl:global-context-item element; if present this element defines constraints on the existence and type of the global context item.

   3. Zero or more xsl:include and xsl:import declarations, which define additional stylesheet modules to be incorporated into this package.

   4. Zero or more ordinary declarations, that is, elements that are permitted as children of xsl:stylesheet or xsl:transform. One possible coding style is to include in the package manifest just a single xsl:import or xsl:include declaration as a reference to the effective top-level stylesheet module; this approach is particularly suitable when writing code that is required to run under releases of XSLT earlier than 3.0. Another approach is to include the substance of the top-level stylesheet module inline within the package manifest.

<xsl:package
  name="http://example.org/complex-arithmetic.xsl"
  package-version="1.0"
  version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:f="http://example.org/complex-arithmetic.xsl">
  
  <xsl:function name="f:complex-number" 
                as="map(xs:integer, xs:double)" visibility="public">
    <xsl:param name="real" as="xs:double"/>
    <xsl:param name="imaginary" as="xs:double"/>
    <xsl:sequence select="{ 0: $real, 1: $imaginary }"/>
  </xsl:function>
  
  <xsl:function name="f:real" 
                as="xs:double" visibility="public">
    <xsl:param name="complex" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="$complex(0)"/>
  </xsl:function>
  
  <xsl:function name="f:imag" 
                as="xs:double" visibility="public">
    <xsl:param name="complex" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="$complex(1)"/>
  </xsl:function>
  
  <xsl:function name="f:add" 
                as="map(xs:integer, xs:double)" visibility="public">
    <xsl:param name="x" as="map(xs:integer, xs:double)"/>
    <xsl:param name="y" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="
         f:complex-number(
           f:real($x) + f:real($y), 
           f:imag($x) + f:imag($y))"/>
  </xsl:function>
  
  <xsl:function name="f:multiply" 
                as="map(xs:integer, xs:double)" visibility="public">
    <xsl:param name="x" as="map(xs:integer, xs:double)"/>
    <xsl:param name="y" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="
         f:complex-number(
           f:real($x)*f:real($y) - f:imag($x)*f:imag($y),
           f:real($x)*f:imag($y) + f:imag($x)*f:real($y))"/>
  </xsl:function>
  
  <!-- etc. -->
  
</xsl:package>

A package that does not itself expose any components may be written using a simplified syntax: the xsl:package element is omitted, and the xsl:stylesheet or xsl:transform element is now the outermost element of the stylesheet module. For compatibility reasons, all the named templates and modes declared in the package are made public. More formally, the principal stylesheet module of the top-level package may be expressed as an xsl:stylesheet or xsl:transform element, which is equivalent to the package represented by the output of the following transformation, preserving the base URI of the source:

 <xsl:transform version="3.0" 
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:t="http://www.w3.org/1999/XSL/TransformAlias">
 
    <xsl:namespace-alias stylesheet-prefix="t" result-prefix="xsl"/>
    
    <xsl:template match="xsl:stylesheet|xsl:transform">
      <t:package declared-modes="no">
        <xsl:copy-of select="@*"/>
        <t:expose component="mode" names="*" visibility="public"/>
        <t:expose component="template" names="*" visibility="public"/>
        <xsl:copy-of select="node()"/>
      </t:package>
    </xsl:template>
 </xsl:transform>

The effect of the input-type-annotations attribute is defined in 4.2.1 Stripping Type Annotations from a Source Tree.

A more extensive example of a package, illustrating how components in a package can be overridden in a client package, is given in 3.4.8 Worked Example of a Library Package.

        PackageVersion   ::= NumericPart ( "-" NamePart )?
        NumericPart      ::= IntegerLiteral ( "." IntegerLiteral )*
        NamePart         ::= NCName

0-rc1 < 0-rc2 < 0 < 1 = 1.0 < 1.0.2 
   < 1.0.3-rc1 < 1.0.3 < 1.0.3.2 < 1.0.10

        PackageVersionRange    ::=  AnyVersion | VersionRanges
        AnyVersion             ::=  "*"
        VersionRanges          ::=  VersionRange (S? "," S? VersionRange)*
        VersionRange           ::=  PackageVersion | VersionPrefix | 
                                      VersionFrom | VersionTo | VersionFromTo
        VersionPrefix          ::=  PackageVersion ".*"
        VersionFrom            ::=  PackageVersion "+"
        VersionTo              ::=  "to" S (PackageVersion | VersionPrefix)
        VersionFromTo          ::=  PackageVersion S 
                                      "to" S (PackageVersion | VersionPrefix)

<xsl:use-package name="Q" version="3.0" xmlns:saxon="http://saxon.sf.net/"/>
     <package-location href="q-dev.xsl" use-when="$diagnostics-on"/>
     <package-location _href="file:/{$prod-path}/q.zip" path-in-archive="q.sef" 
           is-priority="true" format="saxon:sef"/>
     <package-location href="file:/D:/fallback/q.xsl" format="xslt" is-priority="false"/>
</xsl:package>

<xsl:package name="Q"
        version="3.0"                
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:variable name="A" visibility="final" select="$B + 1"/>
  <xsl:variable name="B" visibility="private" select="$C * 2"/>
  <xsl:variable name="C" visibility="public" select="22"/>
</xsl:package>

<xsl:package name="P"
        version="3.0"                
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:use-package name="Q">
    <xsl:override>
      <xsl:variable name="C" visibility="private" select="$xsl:original + 3"/>
    </xsl:override>
  </xsl:use-package>
  
  <xsl:template name="T" visibility="public">
    <xsl:value-of select="$A"/>
  </xsl:template>
</xsl:package>

<xsl:variable name="v" as="xs:integer" visibility="public" select="3"/>

<xsl:function name="f:factory" as="fn(*)" visibility="final">
  <xsl:sequence select="fn() { $v }"/>
</xsl:function>

      
<xsl:use-package name="P">
  <xsl:override>
    <xsl:variable name="v" as="xs:integer" select="4"/>
  </xsl:override>
</xsl:use-package>

<xsl:template name="xsl:initial-template">
  <v value="{f:factory()()}"/>
</xsl:template>

3.5 Stylesheet Modules

[Definition: A package consists of one or more stylesheet modules, each one forming all or part of an XML document.][Definition: [Definition: A package consists of one or more stylesheet modules, each one forming all or part of an XML document.][Definition: [Definition: [Definition: A package consists of one or more stylesheet modules, each one forming all or part of an XML document.]

The principal stylesheet module of a package may take one of three forms:

• A package manifest, as described in 3.4 Packages, which is a subtree rooted at an xsl:package element

• An implicit package, which is a subtree rooted at an xsl:stylesheet or xsl:transform element. This is transformed automatically to a package as described in 3.4 Packages.

• A simplified stylesheet, which is a subtree rooted at a literal result element, as described in 3.7 Simplified Stylesheet Modules. This is first converted to an implicit package by wrapping it in an xsl:stylesheet element using the transformation described in 3.7 Simplified Stylesheet Modules, and then to an explicit package (rooted at an xsl:package element) using the transformation described in 3.4 Packages.

A stylesheet module other than the principal stylesheet module of a package may take either of two forms:

• [Definition: A standard stylesheet module, which is a subtree rooted at an xsl:stylesheet or xsl:transform element.][Definition: [Definition: A standard stylesheet module, which is a subtree rooted at an xsl:stylesheet or xsl:transform element.][Definition: [Definition: [Definition: A standard stylesheet module, which is a subtree rooted at an xsl:stylesheet or xsl:transform element.]

• [Definition: A simplified stylesheet, which is a subtree rooted at a literal result element, as described in 3.7 Simplified Stylesheet Modules. This is first converted to a standard stylesheet module by wrapping it in an xsl:stylesheet element using the transformation described in 3.7 Simplified Stylesheet Modules.][Definition: [Definition: A simplified stylesheet, which is a subtree rooted at a literal result element, as described in 3.7 Simplified Stylesheet Modules. This is first converted to a standard stylesheet module by wrapping it in an xsl:stylesheet element using the transformation described in 3.7 Simplified Stylesheet Modules.][Definition: [Definition: [Definition: A simplified stylesheet, which is a subtree rooted at a literal result element, as described in 3.7 Simplified Stylesheet Modules. This is first converted to a standard stylesheet module by wrapping it in an xsl:stylesheet element using the transformation described in 3.7 Simplified Stylesheet Modules.]

Whichever of the above forms a module takes, the outermost element (xsl:package, xsl:stylesheet, or a literal result element) may either be the outermost element of an XML document, or it may be a child of some (non-XSLT) element in a host document.

[Definition: A stylesheet module whose outermost element is the child of a non-XSLT element in a host document is referred to as an embedded stylesheet module.][Definition: [Definition: A stylesheet module whose outermost element is the child of a non-XSLT element in a host document is referred to as an embedded stylesheet module.][Definition: [Definition: [Definition: A stylesheet module whose outermost element is the child of a non-XSLT element in a host document is referred to as an embedded stylesheet module.]

3.6 Stylesheet Element

<xsl:stylesheet
  id? = id
  version = decimal
  default-mode? = eqname | "#unnamed"〔'#unnamed'〕
  default-validation? = "preserve" | "strip"〔'strip'〕
  input-type-annotations? = "preserve" | "strip" | "unspecified"〔'unspecified'〕
  default-collation? = uris
  extension-element-prefixes? = prefixes
  exclude-result-prefixes? = prefixes
  expand-text? = boolean
  fixed-namespaces? = string
  main-module? = uri
  schema-role? = ncname
  use-when? = expression〔true()〕
  xpath-default-namespace? = uri >
  <!-- Content: (declarations) -->
</xsl:stylesheet>

<xsl:transform
  id? = id
  version = decimal
  default-mode? = eqname | "#unnamed"〔'#unnamed'〕
  default-validation? = "preserve" | "strip"〔'strip'〕
  input-type-annotations? = "preserve" | "strip" | "unspecified"〔'unspecified'〕
  default-collation? = uris
  extension-element-prefixes? = prefixes
  exclude-result-prefixes? = prefixes
  expand-text? = boolean〔'no'〕
  fixed-namespaces? = string
  main-module? = uri
  schema-role? = ncname
  use-when? = expression〔true()〕
  xpath-default-namespace? = uri >
  <!-- Content: (declarations) -->
</xsl:transform>

A stylesheet module is represented by an xsl:stylesheet element in an XML document. xsl:transform is allowed as a synonym for xsl:stylesheet; everything this specification says about the xsl:stylesheet element applies equally to xsl:transform.

The version attribute indicates the version of XSLT that the stylesheet module requires. The attribute is required.

[ERR XTSE0110] The value of the version attribute must be a number: specifically, it must be a valid instance of the type xs:decimal as defined in [XML Schema Part 2].

The version attribute is intended to indicate the version of the XSLT specification against which the stylesheet is written. In a stylesheet written to use XSLT 4.0, the value should normally be set to 4.0. If the value is numerically less than 4.0, the stylesheet is processed using the rules for backwards compatible behavior (see 3.8 Backwards Compatible Processing). If the value is numerically greater than 4.0, the stylesheet is processed using the rules for forwards compatible behavior (see 3.9 Forwards Compatible Processing).

The effect of the input-type-annotations attribute is described in 4.2.1 Stripping Type Annotations from a Source Tree.

The [xsl:]default-validation attribute defines the default value of the validation attribute of all relevant instructions appearing within its scope. For details of the effect of this attribute, see 25.4 Validation.

The optional main-module attribute is purely documentary. By including this attribute in every stylesheet module of a package, an XSLT editing tool may be enabled to locate the principal stylesheet module of the relevant package, and thus to gather information about all the global variables, templates, and functions available within the module being edited. This information can be used (for example) to enable auto-completion and error highlighting of the code as it is entered. Note that it may be inconvenient or misleading to use this attribute when the stylesheet module module is used as a shared component within multiple packages.

[ERR XTSE0120] An xsl:stylesheet, xsl:transform, or xsl:package element must not have any text node children. (This rule applies after stripping of whitespace text nodes as described in 3.11.1 Stripping Whitespace and Commentary from the Stylesheet.)

[Definition: An element occurring as a child of an xsl:package, xsl:stylesheet, xsl:transform, or xsl:override element is called a top-level element.][Definition: [Definition: An element occurring as a child of an xsl:package, xsl:stylesheet, xsl:transform, or xsl:override element is called a top-level element.][Definition: [Definition: [Definition: An element occurring as a child of an xsl:package, xsl:stylesheet, xsl:transform, or xsl:override element is called a top-level element.]

[Definition: Top-level elements fall into two categories: declarations, and user-defined data elements. Top-level elements whose names are in the XSLT namespace are declarations. Top-level elements in any other namespace are user-defined data elements (see 3.6.4 User-defined Data Elements)].[Definition: [Definition: Top-level elements fall into two categories: declarations, and user-defined data elements. Top-level elements whose names are in the XSLT namespace are declarations. Top-level elements in any other namespace are user-defined data elements (see 3.6.4 User-defined Data Elements)].[Definition: [Definition: [Definition: Top-level elements fall into two categories: declarations, and user-defined data elements. Top-level elements whose names are in the XSLT namespace are declarations. Top-level elements in any other namespace are user-defined data elements (see 3.6.4 User-defined Data Elements)].

The declaration elements permitted in the xsl:stylesheet element are:

xsl:accumulator
xsl:attribute-set
xsl:character-map
xsl:decimal-format
xsl:function
xsl:global-context-item
xsl:import
xsl:import-schema
xsl:include
xsl:item-type
xsl:key
xsl:mode
xsl:namespace-alias
xsl:output
xsl:param
xsl:preserve-space
xsl:record-type
xsl:strip-space
xsl:template
xsl:use-package
xsl:variable

Note that the xsl:variable and xsl:param elements can act either as declarations or as instructions. A global variable or parameter is defined using a declaration; a local variable or parameter using an instruction.

The child elements of the xsl:stylesheet element may appear in any order. In most cases, the ordering of these elements does not affect the results of the transformation; however:

• As described in 6.6 Conflict Resolution for Template Rules, when two template rules with the same priority match the same nodes, there are situations where the order of the template rules will affect which is chosen.

• Forwards references to static variables are not allowed in static expressions.

3.8 Backwards Compatible Processing

[Definition: The effective version of an element in a stylesheet module or package manifest is the decimal value of the [xsl:]version attribute (see 3.3 Standard Attributes) on that element or on the innermost ancestor element that has such an attribute, subject to special rules for the xsl:output and xsl:fallback elements.][Definition: [Definition: The effective version of an element in a stylesheet module or package manifest is the decimal value of the [xsl:]version attribute (see 3.3 Standard Attributes) on that element or on the innermost ancestor element that has such an attribute, subject to special rules for the xsl:output and xsl:fallback elements.][Definition: [Definition: [Definition: The effective version of an element in a stylesheet module or package manifest is the decimal value of the [xsl:]version attribute (see 3.3 Standard Attributes) on that element or on the innermost ancestor element that has such an attribute, subject to special rules for the xsl:output and xsl:fallback elements.]

These rules do not apply to the xsl:output element, whose version attribute has an entirely different purpose: it is used to define the version of the output method to be used for serialization. The effective version of an xsl:output element is the effective version of its parent element.

There are additional rules for an xsl:fallback element: see 3.9 Forwards Compatible Processing.

[Definition: An element is processed with backwards compatible behavior if its effective version is less than 4.0.][Definition: [Definition: An element is processed with backwards compatible behavior if its effective version is less than 4.0.][Definition: [Definition: [Definition: An element is processed with backwards compatible behavior if its effective version is less than 4.0.]

Specifically:

• If the effective version is equal to 1.0, then the element is processed with XSLT 1.0 behavior as described in 3.8.1 XSLT 1.0 Compatibility Mode.

• If the effective version is equal to 2.0, then the element is processed with XSLT 2.0 behavior as described in 3.8.2 XSLT 2.0 Compatibility Mode.

• If the effective version is equal to 3.0, then the element is processed with XSLT 3.0 behavior as described in 3.8.3 XSLT 3.0 Compatibility Mode.

• If the effective version is any other value less than 4.0, the recommended action is to raise a static error; however, processors may recognize such values and process the element in an implementation-defined way.

  Note:

  XSLT 1.0 allowed the version attribute to take any decimal value, and invoked forwards compatible processing for any value other than 1.0. XSLT 2.0 allowed the attribute to take any decimal value, and invoked backwards compatible (i.e. 1.0-compatible) processing for any value less than 2.0. Some stylesheets may therefore be encountered that use values other than 1.0 or 2.0. In particular, the value 1.1 is sometimes encountered, as it was used at one stage in a draft language proposal.

It is implementation-defined whether a particular XSLT 4.0 implementation supports backwards compatible behavior for any XSLT version earlier than XSLT 4.0.

[ERR XTDE0160] It is a dynamic error if an element has an effective version of V (with V < 4.0) when the implementation does not support backwards compatible behavior for XSLT version V.

3.9 Forwards Compatible Processing

The intent of forwards compatible behavior is to make it possible to write a stylesheet that takes advantage of features introduced in some version of XSLT subsequent to XSLT 4.0, while retaining the ability to execute the stylesheet with an XSLT 4.0 processor using appropriate fallback behavior.

It is always possible to write conditional code to run under different XSLT versions by using the use-when feature described in 3.11.3 Conditional Element Inclusion. The rules for forwards compatible behavior supplement this mechanism in two ways:

• Certain constructs in the stylesheet that mean nothing to an XSLT 4.0 processor are ignored, rather than being treated as errors.

• Explicit fallback behavior can be defined for instructions defined in a future XSLT release, using the xsl:fallback instruction.

The detailed rules follow.

[Definition: An element is processed with forwards compatible behavior if its effective version is greater than 4.0.][Definition: [Definition: An element is processed with forwards compatible behavior if its effective version is greater than 4.0.][Definition: [Definition: [Definition: An element is processed with forwards compatible behavior if its effective version is greater than 4.0.]

These rules do not apply to the version attribute of the xsl:output element, which has an entirely different purpose: it is used to define the version of the output method to be used for serialization.

When an element is processed with forwards compatible behavior:

• If the element is in the XSLT namespace and appears as a child of the xsl:stylesheet element, and XSLT 4.0 does not allow the element to appear as a child of the xsl:stylesheet element, then the element and its content must be ignored.

• If the element has an attribute that XSLT 4.0 does not allow the element to have, then the attribute must be ignored.

• If the element is in the XSLT namespace and appears as a child of an element whose content model requires a sequence constructor, and XSLT 4.0 does not allow such elements to appear as part of a sequence constructor, then:

  1. If the element has one or more xsl:fallback children, then no error is raised either statically or dynamically, and the result of evaluating the instruction is the concatenation of the sequences formed by evaluating the sequence constructors within its xsl:fallback children, in document order. Siblings of the xsl:fallback elements are ignored, even if they are valid XSLT 4.0 instructions.

  2. If the element has no xsl:fallback children, then a static error is raised in the same way as if forwards compatible behavior were not enabled.

<xsl:stylesheet version="17.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:exciting-new-17.0-feature>
      <xsl:fly-to-the-moon/>
      <xsl:fallback>
        <html>
          <head>
            <title>XSLT 17.0 required</title>
          </head>
          <body>
            <p>Sorry, this stylesheet requires XSLT 17.0.</p>
          </body>
        </html>
      </xsl:fallback>
    </xsl:exciting-new-17.0-feature>
  </xsl:template>
</xsl:stylesheet>

<xsl:stylesheet version="18.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:important-new-17.0-declaration/>

  <xsl:template match="/">
    <xsl:choose>
      <xsl:when test="number(system-property('xsl:version')) lt 17.0">
        <xsl:message terminate="yes">
          <xsl:text>Sorry, this stylesheet requires XSLT 17.0.</xsl:text>
        </xsl:message>
      </xsl:when>
      <xsl:otherwise>
        ...
      </xsl:otherwise>
    </xsl:choose>
  </xsl:template>
  ...
</xsl:stylesheet>

For an XSLT 4.0 processor, the effective version of an xsl:fallback element that has no explicit version attribute is 4.0: more generally, it is the version of XSLT supported by the processor. This rule is designed to ensure that the xsl:fallback element itself is not processed with forwards compatible behavior, which would be pointless since the whole purpose of the instruction is to provide code that can be evaluated with an earlier version of XSLT.

3.10 Combining Stylesheet Modules

XSLT provides two mechanisms to construct a package from multiple stylesheet modules:

• an inclusion mechanism that allows stylesheet modules to be combined without changing the semantics of the modules being combined, and

• an import mechanism that allows stylesheet modules to override each other.

<xsl:stylesheet version="3.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:import href="article.xsl"/>
  <xsl:import href="bigfont.xsl"/>
  <xsl:attribute-set name="note-style">
    <xsl:attribute name="font-style">italic</xsl:attribute>
  </xsl:attribute-set>
</xsl:stylesheet>

4.2 Preprocessing Source Documents

Source documents supplied as input to a transformation may be subject to preprocessing. Two kinds of preprocessing are defined: stripping of type annotations (see 4.2.1 Stripping Type Annotations from a Source Tree), and stripping of whitespace text nodes (see 4.2.2 Stripping Whitespace from a Source Tree).

Stripping of type annotations happens before stripping of whitespace text nodes.

The source documents to which this applies are as follows:

• The document containing the global context item if it is a node;

• Any documents containing a node present in the initial match selection;

• Any document containing a node that is returned by the functions document, doc, or collection;

• Any document read using xsl:source-document.

If a node other than a document node is supplied (for example as the global context item), then the preprocessing is applied to the entire document containing that node. If several nodes within the same document are supplied (for example as nodes in the initial match selection, or as nodes returned by the collection function), then the preprocessing is only applied to that document once. If a whitespace text node is supplied (for example as the global context item) and the rules cause this node to be stripped from its containing tree, then the behavior is as if this node had not been supplied (which may cause an error, for example if a global context item is required.)

The rules determining whether or not stripping of annotations and/or whitespace happens are defined at the level of a package. Declarations within a library package only affect the handling of documents loaded using a call on the document, doc, or collection functions or an evaluation of an xsl:source-document instruction appearing lexically within the same package. Declarations within the top-level package also affect the processing of the global context item and the initial match selection.

The semantics of the document, doc, and collection functions are formally defined in terms of mappings from URIs to document nodes maintained within the dynamic context (see 5.3.3 Initializing the Dynamic Context). The effect of the declarations that control stripping of type annotations and whitespace is therefore to modify this mapping (so it now maps the URI to a stripped document). The modification applies to the dynamic context for calls to these function appearing within a particular package; each package therefore has a different set of mappings. This means that when two calls to the doc function appear in different packages, specifying the same absolute URI, then in general different documents are returned. An implementation may return the same document for two such calls if it is able to determine that the effect of the annotation and whitespace stripping rules in both packages is the same.