XSLT is used to describe rules for transforming a source tree into a result tree. The transformation is achieved by associating patterns with templates contained within a stylesheet. An XSLT processor matches patterns against elements in the source tree and instantiates associated templates to create various parts of the result tree. While constructing the result tree, the processor might filter and reorder elements from the source tree and add arbitrary structure, as specified by the XSLT transformation. The result tree is separate from the source tree—that is, no nodes are shared between the two—and its structure can be completely different from the structure of the source tree.XSLT stylesheets are well-formed XML documents that contain elements and attributes in the XSLT namespace (http://www.w3.org/1999/XSL/Transform). These elements and attributes are used to provide instructions to an XSLT processor regarding the transformation it needs to effect. A stylesheet may also contain elements and attributes that are not in the XSLT namespace. XSLT processors typically interpret these as markup to be directly added to the result tree. An exception is the case of extension-elements, which serve as XSLT processor instructions and are in a separate namespace defined using a special mechanism.XSLT instructions are contained within templates. The result tree is constructed by finding the template rule for the root node and instantiating its template. When a template is instantiated, each instruction within it is executed and replaced by the result tree fragment it creates. These instructions may select and process descendant source elements, which may entail instantiating other templates. It is important to note that elements are only processed when they have been selected by the execution of an instruction. XSLT uses the XPath language to select elements from the source tree and to do conditional processing.An XSLT processor instantiates templates based on pattern-matching rules. In the process of finding an applicable template rule, more than one template rule may have a pattern that matches a given element. The processor then uses special conflict-resolution logic (which uses template priorities) to ensure the best template rule is applied.
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