It started with XSL and ended up with XSLT, XPath, and XSL-FO.

XSL stands for EXtensible Stylesheet Language.

The World Wide Web Consortium (W3C) started to develop XSL because there was a need for an XML-based Stylesheet Language.

HTML uses predefined tags and the meaning of the tags are well understood.

The <table> element in HTML defines a table - and a browser knows how to display it.

Adding styles to HTML elements is simple. Telling a browser to display an element in a special font or color, is easy with CSS.

XML does not use predefined tags (we can use any tag-names we like), and the meaning of these tags are not well understood.

A <table> element could mean an HTML table, a piece of furniture, or something else - and a browser does not know how to display it.

XSL describes how the XML document should be displayed!

XSL consists of three parts:

XSLT (Extensible Stylesheet Language Transformations) is a declarative, XML-based language used for the transformation of XML documents into other XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized (output) by the processor in standard XML syntax or in another format, such as HTML or plain text. XSLT is often used to convert XML data into HTML or XHTML documents for display as a web page: the transformation may happen dynamically either on the client or on the server, or it may be done as part of the publishing process. It is also used to create output for printing or direct video display, typically by transforming the original XML into XSL Formatting Objects to create formatted output which can then be converted to a variety of formats, a few of which are PDF, PostScript and PNG. XSLT is also used to translate XML messages between different XML schemas, or to make changes to documents within the scope of a single schema, for example by removing the parts of a message that are not needed.