We’ll be working with this document in the following XPath and XSLT examples. XSLT Programs and XPath Expressions XSLT (eXtensible Stylesheet Language for Transformations) is a programming language for transforming XML data. For historical reasons, it is called a stylesheet language, and XSLT programs are frequently called stylesheets.
The most important difference between them, from which more-specific differences follow, is that an XHTML document can be checked for well-formedness and parsed in the absence of a grammar. We have expressed this difference by saying that elements must form a tree in all SGML languages as well as in all XML languages, but, in XML languages, additionally, the document’s markup must explicitly show the tree structure. HTML and XHTML have several other, less important differences, such as XML is case sensitive and XHTML tags are defined to be in lowercase.