Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. An XML template engine (or XML template processor) is a specialized template processor for XML input and/or output, working in an XML template system context. XSLT processors may be delivered as standalone applications, or as software components or libraries intended for use by applications. Many web browsers and web server software have XSLT processor components built into them. Most current operating systems have an XSLT processor installed. For example, Windows XP comes with the MSXML3 library, which includes an XSLT processor. Early XSLT processors had very few optimizations; stylesheet documents were read using the Document Object Model and the processor would act on them directly. XPath engines were also not optimized.