The XSLT namespace has the URI &XSLT.ns;.

XSLT processors must use the XML namespaces mechanism to recognize elements and attributes from this namespace. Elements from the XSLT namespace are recognized only in the stylesheet not in the source document. The complete list of XSLT-defined elements is specified in . Vendors must not extend the XSLT namespace with additional elements or attributes. Instead, any extension must be in a separate namespace. Any namespace that is used for additional instruction elements must be identified by means of the extension element mechanism specified in .

This specification uses a prefix of xsl: for referring to elements in the XSLT namespace. However, XSLT stylesheets are free to use any prefix, provided that there is a namespace declaration that binds the prefix to the URI of the XSLT namespace.

An element from the XSLT namespace may have any attribute not from the XSLT namespace, provided that the expanded-name of the attribute has a non-null namespace URI. The presence of such attributes must not change the behavior of XSLT elements and functions defined in this document. Thus, an XSLT processor is always free to ignore such attributes, and must ignore such attributes without giving an error if it does not recognize the namespace URI. Such attributes can provide, for example, unique identifiers, optimization hints, or documentation.

It is an error for an element from the XSLT namespace to have attributes with expanded-names that have null namespace URIs (i.e. attributes with unprefixed names) other than attributes defined for the element in this document.

A stylesheet is represented by an xsl:stylesheet element in an XML document. xsl:transform is allowed as a synonym for xsl:stylesheet.

An xsl:stylesheet element must have a version attribute, indicating the version of XSLT that the stylesheet requires. For this version of XSLT, the value should be 1.0. When the value is not equal to 1.0, forwards-compatible processing mode is enabled (see ).

The xsl:stylesheet element may contain the following types of elements:

An element occurring as a child of an xsl:stylesheet element is called a top-level element.

This example shows the structure of a stylesheet. Ellipses (...) indicate where attribute values or content have been omitted. Although this example shows one of each type of allowed element, stylesheets may contain zero or more of each of these elements.

The order in which the children of the xsl:stylesheet element occur is not significant except for xsl:import elements and for error recovery. Users are free to order the elements as they prefer, and stylesheet creation tools need not provide control over the order in which the elements occur.

In addition, the xsl:stylesheet element may contain any element not from the XSLT namespace, provided that the expanded-name of the element has a non-null namespace URI. The presence of such top-level elements must not change the behavior of XSLT elements and functions defined in this document; for example, it would not be permitted for such a top-level element to specify that xsl:apply-templates was to use different rules to resolve conflicts. Thus, an XSLT processor is always free to ignore such top-level elements, and must ignore a top-level element without giving an error if it does not recognize the namespace URI. Such elements can provide, for example,

A simplified syntax is allowed for stylesheets that consist of only a single template for the root node. The stylesheet may consist of just a literal result element (see ). Such a stylesheet is equivalent to a stylesheet with an xsl:stylesheet element containing a template rule containing the literal result element; the template rule has a match pattern of /. For example

has the same meaning as

A literal result element that is the document element of a stylesheet must have an xsl:version attribute, which indicates the version of XSLT that the stylesheet requires. For this version of XSLT, the value should be 1.0; the value must be a Number. Other literal result elements may also have an xsl:version attribute. When the xsl:version attribute is not equal to 1.0, forwards-compatible processing mode is enabled (see ).

The allowed content of a literal result element when used as a stylesheet is no different from when it occurs within a stylesheet. Thus, a literal result element used as a stylesheet cannot contain top-level elements.

In some situations, the only way that a system can recognize that an XML document needs to be processed by an XSLT processor as an XSLT stylesheet is by examining the XML document itself. Using the simplified syntax makes this harder.

Therefore, the simplified syntax should not be used for XSLT stylesheets that may be used in such a situation. This situation can, for example, arise when an XSLT stylesheet is transmitted as a message with a MIME media type of text/xml or application/xml to a recipient that will use the MIME media type to determine how the message is processed.

The name of an internal XSLT object, specifically a named template (see ), a mode (see ), an attribute set (see ), a key (see ), a decimal-format (see ), a variable or a parameter (see ) is specified as a QName. If it has a prefix, then the prefix is expanded into a URI reference using the namespace declarations in effect on the attribute in which the name occurs. The expanded-name consisting of the local part of the name and the possibly null URI reference is used as the name of the object. The default namespace is not used for unprefixed names.

An element enables forwards-compatible mode for itself, its attributes, its descendants and their attributes if either it is an xsl:stylesheet element whose version attribute is not equal to 1.0, or it is a literal result element that has an xsl:version attribute whose value is not equal to 1.0, or it is a literal result element that does not have an xsl:version attribute and that is the document element of a stylesheet using the simplified syntax (see ). A literal result element that has an xsl:version attribute whose value is equal to 1.0 disables forwards-compatible mode for itself, its attributes, its descendants and their attributes.

If an element is processed in forwards-compatible mode, then:

Thus, any XSLT 1.0 processor must be able to process the following stylesheet without error, although the stylesheet includes elements from the XSLT namespace that are not defined in this specification:

If an expression occurs in an attribute that is processed in forwards-compatible mode, then an XSLT processor must recover from errors in the expression as follows:

XSLT provides two mechanisms to combine stylesheets:

Normally an XSLT stylesheet is a complete XML document with the xsl:stylesheet element as the document element. However, an XSLT stylesheet may also be embedded in another resource. Two forms of embedding are possible:

To facilitate the second form of embedding, the xsl:stylesheet element is allowed to have an ID attribute that specifies a unique identifier.

The following example shows how the xml-stylesheet processing instruction can be used to allow a document to contain its own stylesheet. The URI reference uses a relative URI with a fragment identifier to locate the xsl:stylesheet element:

The normal restrictions on the children of the root node are relaxed for the result tree. The result tree may have any sequence of nodes as children that would be possible for an element node. In particular, it may have text node children, and any number of element node children. When written out using the XML output method (see ), it is possible that a result tree will not be a well-formed XML document; however, it will always be a well-formed external general parsed entity.

When the source tree is created by parsing a well-formed XML document, the root node of the source tree will automatically satisfy the normal restrictions of having no text node children and exactly one element child. When the source tree is created in some other way, for example by using the DOM, the usual restrictions are relaxed for the source tree as for the result tree.

Every node also has an associated URI called its base URI, which is used for resolving attribute values that represent relative URIs into absolute URIs. If an element or processing instruction occurs in an external entity, the base URI of that element or processing instruction is the URI of the external entity; otherwise, the base URI is the base URI of the document. The base URI of the document node is the URI of the document entity. The base URI for a text node, a comment node, an attribute node or a namespace node is the base URI of the parent of the node.

The root node has a mapping that gives the URI for each unparsed entity declared in the document's DTD. The URI is generated from the system identifier and public identifier specified in the entity declaration. The XSLT processor may use the public identifier to generate a URI for the entity instead of the URI specified in the system identifier. If the XSLT processor does not use the public identifier to generate the URI, it must use the system identifier; if the system identifier is a relative URI, it must be resolved into an absolute URI using the URI of the resource containing the entity declaration as the base URI .

After the tree for a source document or stylesheet document has been constructed, but before it is otherwise processed by XSLT, some text nodes are stripped. A text node is never stripped unless it contains only whitespace characters. Stripping the text node removes the text node from the tree. The stripping process takes as input a set of element names for which whitespace must be preserved. The stripping process is applied to both stylesheets and source documents, but the set of whitespace-preserving element names is determined differently for stylesheets and for source documents.

A text node is preserved if any of the following apply:

Otherwise, the text node is stripped.

The xml:space attributes are not stripped from the tree.

For stylesheets, the set of whitespace-preserving element names consists of just xsl:text.

For source documents, the set of whitespace-preserving element names is specified by xsl:strip-space and xsl:preserve-space top-level elements. These elements each have an elements attribute whose value is a whitespace-separated list of NameTests. Initially, the set of whitespace-preserving element names contains all element names. If an element name matches a NameTest in an xsl:strip-space element, then it is removed from the set of whitespace-preserving element names. If an element name matches a NameTest in an xsl:preserve-space element, then it is added to the set of whitespace-preserving element names. An element matches a NameTest if and only if the NameTest would be true for the element as an XPath node test. Conflicts between matches to xsl:strip-space and xsl:preserve-space elements are resolved the same way as conflicts between template rules (see ). Thus, the applicable match for a particular element name is determined as follows:

It is an error if this leaves more than one match. An XSLT processor may signal the error; if it does not signal the error, it must recover by choosing, from amongst the matches that are left, the one that occurs last in the stylesheet.

A list of source nodes is processed to create a result tree fragment. The result tree is constructed by processing a list containing just the root node. A list of source nodes is processed by appending the result tree structure created by processing each of the members of the list in order. A node is processed by finding all the template rules with patterns that match the node, and choosing the best amongst them; the chosen rule's template is then instantiated with the node as the current node and with the list of source nodes as the current node list. A template typically contains instructions that select an additional list of source nodes for processing. The process of matching, instantiation and selection is continued recursively until no new source nodes are selected for processing.

Implementations are free to process the source document in any way that produces the same result as if it were processed using this processing model.

Template rules identify the nodes to which they apply by using a pattern. As well as being used in template rules, patterns are used for numbering (see ) and for declaring keys (see ). A pattern specifies a set of conditions on a node. A node that satisfies the conditions matches the pattern; a node that does not satisfy the conditions does not match the pattern. The syntax for patterns is a subset of the syntax for expressions. In particular, location paths that meet certain restrictions can be used as patterns. An expression that is also a pattern always evaluates to an object of type node-set. A node matches a pattern if the node is a member of the result of evaluating the pattern as an expression with respect to some possible context; the possible contexts are those whose context node is the node being matched or one of its ancestors.

Here are some examples of patterns:

A pattern must match the grammar for Pattern. A Pattern is a set of location path patterns separated by |. A location path pattern is a location path whose steps all use only the child or attribute axes. Although patterns must not use the descendant-or-self axis, patterns may use the // operator as well as the / operator. Location path patterns can also start with an id or key function call with a literal argument. Predicates in a pattern can use arbitrary expressions just like predicates in a location path.

A pattern is defined to match a node if and only if there is possible context such that when the pattern is evaluated as an expression with that context, the node is a member of the resulting node-set. When a node is being matched, the possible contexts have a context node that is the node being matched or any ancestor of that node, and a context node list containing just the context node.

For example, p matches any p element, because for any p if the expression p is evaluated with the parent of the p element as context the resulting node-set will contain that p element as one of its members.

Although the semantics of patterns are specified indirectly in terms of expression evaluation, it is easy to understand the meaning of a pattern directly without thinking in terms of expression evaluation. In a pattern, | indicates alternatives; a pattern with one or more | separated alternatives matches if any one of the alternative matches. A pattern that consists of a sequence of StepPatterns separated by / or // is matched from right to left. The pattern only matches if the rightmost StepPattern matches and a suitable element matches the rest of the pattern; if the separator is / then only the parent is a suitable element; if the separator is //, then any ancestor is a suitable element. A StepPattern that uses the child axis matches if the NodeTest is true for the node and the node is not an attribute node. A StepPattern that uses the attribute axis matches if the NodeTest is true for the node and the node is an attribute node. When [] is present, then the first PredicateExpr in a StepPattern is evaluated with the node being matched as the context node and the siblings of the context node that match the NodeTest as the context node list, unless the node being matched is an attribute node, in which case the context node list is all the attributes that have the same parent as the attribute being matched and that match the NameTest.

For example

matches a node if and only if all of the following are true:

A template rule is specified with the xsl:template element. The match attribute is a Pattern that identifies the source node or nodes to which the rule applies. The match attribute is required unless the xsl:template element has a name attribute (see ). It is an error for the value of the match attribute to contain a VariableReference. The content of the xsl:template element is the template that is instantiated when the template rule is applied.

For example, an XML document might contain:

The following template rule matches emph elements and produces a fo:inline-sequence formatting object with a font-weight property of bold.

As described next, the xsl:apply-templates element recursively processes the children of the source element.

This example creates a block for a chapter element and then processes its immediate children.

In the absence of a select attribute, the xsl:apply-templates instruction processes all of the children of the current node, including text nodes. However, text nodes that have been stripped as specified in will not be processed. If stripping of whitespace nodes has not been enabled for an element, then all whitespace in the content of the element will be processed as text, and thus whitespace between child elements will count in determining the position of a child element as returned by the position function.

A select attribute can be used to process nodes selected by an expression instead of processing all children. The value of the select attribute is an expression. The expression must evaluate to a node-set. The selected set of nodes is processed in document order, unless a sorting specification is present (see ). The following example processes all of the author children of the author-group:

The following example processes all of the given-names of the authors that are children of author-group:

This example processes all of the heading descendant elements of the book element.

It is also possible to process elements that are not descendants of the current node. This example assumes that a department element has group children and employee descendants. It finds an employee's department and then processes the group children of the department.

Multiple xsl:apply-templates elements can be used within a single template to do simple reordering. The following example creates two HTML tables. The first table is filled with domestic sales while the second table is filled with foreign sales.

It is possible for a source node to match more than one template rule. The template rule to be used is determined as follows:

It is an error if this leaves more than one matching template rule. An XSLT processor may signal the error; if it does not signal the error, it must recover by choosing, from amongst the matching template rules that are left, the one that occurs last in the stylesheet.

A template rule that is being used to override a template rule in an imported stylesheet (see ) can use the xsl:apply-imports element to invoke the overridden template rule.

At any point in the processing of a stylesheet, there is a current template rule. Whenever a template rule is chosen by matching a pattern, the template rule becomes the current template rule for the instantiation of the rule's template. When an xsl:for-each element is instantiated, the current template rule becomes null for the instantiation of the content of the xsl:for-each element.

xsl:apply-imports processes the current node using only template rules that were imported into the stylesheet element containing the current template rule; the node is processed in the current template rule's mode. It is an error if xsl:apply-imports is instantiated when the current template rule is null.

For example, suppose the stylesheet doc.xsl contains a template rule for example elements:

Another stylesheet could import doc.xsl and modify the treatment of example elements as follows:

The combined effect would be to transform an example into an element of the form:

...

Modes allow an element to be processed multiple times, each time producing a different result.

Both xsl:template and xsl:apply-templates have an optional mode attribute. The value of the mode attribute is a QName, which is expanded as described in . If xsl:template does not have a match attribute, it must not have a mode attribute. If an xsl:apply-templates element has a mode attribute, then it applies only to those template rules from xsl:template elements that have a mode attribute with the same value; if an xsl:apply-templates element does not have a mode attribute, then it applies only to those template rules from xsl:template elements that do not have a mode attribute.

There is a built-in template rule to allow recursive processing to continue in the absence of a successful pattern match by an explicit template rule in the stylesheet. This template rule applies to both element nodes and the root node. The following shows the equivalent of the built-in template rule:

There is also a built-in template rule for each mode, which allows recursive processing to continue in the same mode in the absence of a successful pattern match by an explicit template rule in the stylesheet. This template rule applies to both element nodes and the root node. The following shows the equivalent of the built-in template rule for mode m.

There is also a built-in template rule for text and attribute nodes that copies text through:

The built-in template rule for processing instructions and comments is to do nothing.

The built-in template rule for namespace nodes is also to do nothing. There is no pattern that can match a namespace node; so, the built-in template rule is the only template rule that is applied for namespace nodes.

The built-in template rules are treated as if they were imported implicitly before the stylesheet and so have lower import precedence than all other template rules. Thus, the author can override a built-in template rule by including an explicit template rule.

A template can also contain text nodes. Each text node in a template remaining after whitespace has been stripped as specified in will create a text node with the same string-value in the result tree. Adjacent text nodes in the result tree are automatically merged.

Note that text is processed at the tree level. Thus, markup of < in a template will be represented in the stylesheet tree by a text node that includes the character <. This will create a text node in the result tree that contains a < character, which will be represented by the markup < (or an equivalent character reference) when the result tree is externalized as an XML document (unless output escaping is disabled as described in ).

Literal data characters may also be wrapped in an xsl:text element. This wrapping may change what whitespace characters are stripped (see ) but does not affect how the characters are handled by the XSLT processor thereafter.

The xsl:processing-instruction element is instantiated to create a processing instruction node. The content of the xsl:processing-instruction element is a template for the string-value of the processing instruction node. The xsl:processing-instruction element has a required name attribute that specifies the name of the processing instruction node. The value of the name attribute is interpreted as an attribute value template.

For example, this

would create the processing instruction

It is an error if the string that results from instantiating the name attribute is not both an NCName and a PITarget. An XSLT processor may signal the error; if it does not signal the error, it must recover by not adding the processing instruction to the result tree.

It is an error if instantiating the content of xsl:processing-instruction creates nodes other than text nodes. An XSLT processor may signal the error; if it does not signal the error, it must recover by ignoring the offending nodes together with their content.

It is an error if the result of instantiating the content of the xsl:processing-instruction contains the string ?>. An XSLT processor may signal the error; if it does not signal the error, it must recover by inserting a space after any occurrence of ? that is followed by a >.

The xsl:comment element is instantiated to create a comment node in the result tree. The content of the xsl:comment element is a template for the string-value of the comment node.

For example, this

would create the comment

It is an error if instantiating the content of xsl:comment creates nodes other than text nodes. An XSLT processor may signal the error; if it does not signal the error, it must recover by ignoring the offending nodes together with their content.

It is an error if the result of instantiating the content of the xsl:comment contains the string -- or ends with -. An XSLT processor may signal the error; if it does not signal the error, it must recover by inserting a space after any occurrence of - that is followed by another - or that ends the comment.

The xsl:copy element provides an easy way of copying the current node. Instantiating the xsl:copy element creates a copy of the current node. The namespace nodes of the current node are automatically copied as well, but the attributes and children of the node are not automatically copied. The content of the xsl:copy element is a template for the attributes and children of the created node; the content is instantiated only for nodes of types that can have attributes or children (i.e. root nodes and element nodes).

The xsl:copy element may have a use-attribute-sets attribute (see ). This is used only when copying element nodes.

The root node is treated specially because the root node of the result tree is created implicitly. When the current node is the root node, xsl:copy will not create a root node, but will just use the content template.

For example, the identity transformation can be written using xsl:copy as follows:

When the current node is an attribute, then if it would be an error to use xsl:attribute to create an attribute with the same name as the current node, then it is also an error to use xsl:copy (see ).

The following example shows how xml:lang attributes can be easily copied through from source to result. If a stylesheet defines the following named template:

then it can simply do

instead of

when it wants to copy the xml:lang attribute.

Within a template, the xsl:value-of element can be used to compute generated text, for example by extracting text from the source tree or by inserting the value of a variable. The xsl:value-of element does this with an expression that is specified as the value of the select attribute. Expressions can also be used inside attribute values of literal result elements by enclosing the expression in curly braces ({}).

Attribute Value Templates

In an attribute value that is interpreted as an attribute value template, such as an attribute of a literal result element, an expression can be used by surrounding the expression with curly braces ({}). The attribute value template is instantiated by replacing the expression together with surrounding curly braces by the result of evaluating the expression and converting the resulting object to a string as if by a call to the string function. Curly braces are not recognized in an attribute value in an XSLT stylesheet unless the attribute is specifically stated to be one that is interpreted as an attribute value template; in an element syntax summary, the value of such attributes is surrounded by curly braces.

Not all attributes are interpreted as attribute value templates. Attributes whose value is an expression or pattern, attributes of top-level elements and attributes that refer to named XSLT objects are not interpreted as attribute value templates. In addition, xmlns attributes are not interpreted as attribute value templates; it would not be conformant with the XML Namespaces Recommendation to do this.

The following example creates an img result element from a photograph element in the source; the value of the src attribute of the img element is computed from the value of the image-dir variable and the string-value of the href child of the photograph element; the value of the width attribute of the img element is computed from the value of the width attribute of the size child of the photograph element:

/images ]]>

With this source

headquarters.jpg ]]>

the result would be

]]>

When an attribute value template is instantiated, a double left or right curly brace outside an expression will be replaced by a single curly brace. It is an error if a right curly brace occurs in an attribute value template outside an expression without being followed by a second right curly brace. A right curly brace inside a Literal in an expression is not recognized as terminating the expression.

Curly braces are not recognized recursively inside expressions. For example:

]]>

is not allowed. Instead, use simply:

]]>

The xsl:number element is used to insert a formatted number into the result tree. The number to be inserted may be specified by an expression. The value attribute contains an expression. The expression is evaluated and the resulting object is converted to a number as if by a call to the number function. The number is rounded to an integer and then converted to a string using the attributes specified in ; in this context, the value of each of these attributes is interpreted as an attribute value template. After conversion, the resulting string is inserted in the result tree. For example, the following example numbers a sorted list:

If no value attribute is specified, then the xsl:number element inserts a number based on the position of the current node in the source tree. The following attributes control how the current node is to be numbered:

In addition, the attributes specified in are used for number to string conversion, as in the case when the value attribute is specified.

The xsl:number element first constructs a list of positive integers using the level, count and from attributes:

The list of numbers is then converted into a string using the attributes specified in ; in this context, the value of each of these attributes is interpreted as an attribute value template. After conversion, the resulting string is inserted in the result tree.

The following would number the items in an ordered list:

The following two rules would number title elements. This is intended for a document that contains a sequence of chapters followed by a sequence of appendices, where both chapters and appendices contain sections, which in turn contain subsections. Chapters are numbered 1, 2, 3; appendices are numbered A, B, C; sections in chapters are numbered 1.1, 1.2, 1.3; sections in appendices are numbered A.1, A.2, A.3.

The following example numbers notes sequentially within a chapter:

The following example would number H4 elements in HTML with a three-part label: