1. A method comprising: receiving an Extensible Stylesheet Language Transformation (XSLT) style sheet into an XSLT processing environment; receiving an Extensible Markup Language (XML) input document into the XSLT processing environment; compiling the received XSLT style sheet into Extensible Stylesheet Language (XSL) byte code fragments; compiling the XSL byte code fragments into native code when demand for the XSL byte code fragments exceeds a usage threshold, wherein compiling the XSL byte code fragments into native code comprises: statically compiling the XSL byte code fragments into built-in function modules; and compiling the built-in function modules into native code at runtime; interpreting XSL byte codes fragments when demand is below the usage threshold instead of generating native code, wherein the compiling is performed by an XSLT just-in-time (XJIT) compiler in the XSLT processing environment, wherein the native code invokes the built-in function modules via an XJIT native interface (XNI), wherein the XJIT compiler generates the native code from variable instructions associated with the XSLT style sheet, the generated native code having the ability to invoke the built-in functions via an XJIT native interface, and wherein source code of the XJIT compiler includes XML Path Language instructions; and executing the native code to transform the XML input document into one or more XML output documents.