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2 Introduction to XSL Transformation 2.1 Tree Construction 2.2 XSL Namespace

## 2 Introduction to XSL Transformation

### 2.1 Tree Construction

The Tree Construction is described in "XSL Transformations" [XSLT].

The provisions in "XSL Transformations" form an integral part of this recommendation and are considered normative.

### 2.2 XSL Namespace

The XSL namespace has the URI `http://www.w3.org/1999/XSL/Format`.

> NOTE:

> The `1999` in the URI indicates the year in which the URI was allocated by the W3C. It does not indicate the version of XSL being used.

XSL processors must use the XML namespaces mechanism [W3C XML Names] to recognize elements and attributes from this namespace. Elements from the XSL namespace are recognized only in the stylesheet, not in the source document. Implementors must not extend the XSL namespace with additional elements or attributes. Instead, any extension must be in a separate namespace.

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

An element from the XSL namespace may have any attribute not from the XSL 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 XSL elements and functions defined in this document. Thus, an XSL 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 XSL 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.

> NOTE:

> The conventions used for the names of XSL elements, attributes, and functions are as follows: names are all lowercase, hyphens are used to separate words, dots are used to separate names for the components of complex datatypes, and abbreviations are used only if they already appear in the syntax of a related language such as XML or HTML.

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