by Adam Michael Wood
December 12, 2018
December 12, 2018

XSL: Render Your XML Documents With These Critical Tools

XSL: XSLT, XSL-FO and XPath

XSL ( Extensible Stylesheet Language ) is a styling language for XML documents.

Most often when someone is referring to XSL, they are referring to XSLT or XSL-FO. But it also includes XPath and XQuery.

In this guide, we will discuss all of these tools, because without them, XML is really just a simple database.

HTML was designed primarily to present human-readable content on a screen, in a web browser. XML was designed to store and transfer arbitrarily structured content, primarily for consumption by an application, and frequently for presentation on a screen.

But what are you supposed to do if you want to present your XML not on a screen but on printed paper, in a book, catalog, or brochure? Or as a PDF approximation of those formats?

XSLT

XSLT stands for Extensible Stylesheet Language Transformations, and is a part of XSL — along with XSL-FO, XPath, and XQuery

The purpose of XSLT is to provide a language that can specify how to transform an XML document into another document. XSLT can be used to transform XML into another XML with a different document type definition or schema.

It can also be used to transform XML into HTML or into an XML Formatting Objects document.

General XSL and XML Guides and Tutorials

XSLT Tutorials

Additional XSLT Learning Materials

XSLT Reference

Tools for Working With XSLT

XSLT Books

Why You Need XSLT

If you work with XML documents, and need a way to translate in and out of multiple formats, you should spend some time learning the ins and outs of XSL & XSLT.

XSL-FO

HTML has virtually no support for paginated content, and non-HTML versions of XML provide no styling information at all. You can use a print-only CSS stylesheet, but that gives only rudimentary control over the print layout.

The solution starts with the relatively underused standard with huge potential, XSL-FO.

The FO stands for “Formatting Objects,” and the standard is a part of XSL — eXtensible Stylesheet Language. The other two parts of the XSL standard are XSLT and XPath.

How Does XSL-FO Work?

If you are used to HTML and CSS, XSL-FO might seem a little weird. One of the most important ideas with HTML+CSS is separation of content from styling. The HTML document contains all the content, ideally without any styling or presentational information; the CSS sheet provides styling and presentation.

XSL-FO is more like a TeX/LaTeX, PostScript or Microsoft Word format — it is a page-layout format that contains both the content and the styling. An XSL-FO (or .fo ) file can be printed directly with a FO Processor.

This doesn’t break the separation of concerns, though. Typically, one does not author .fo files directly. The standard method is to produce XML or (X)HTML first, either by hand or programmatically (for example, out of a database), and the use XSLT to transform the XML into XSL-FO. XSL-FO can then be consumed by an FO Processor, and turned into printed material for distribution.

XSL-FO, along with the entire XML production chain, is a highly useful tool when you have content that has to be repurposed for different media. The most common use-case is inventory information that has to be displayed in catalogs, brochures, online stores, signage, labels, packaging, and several other formats, all from a single source.

XSL-FO Tutorials

Additional XFL-FO Learning Materials

XSL-FO Tools

XSL-FO Books

  • XSL-FO (2002), by Dave Pawson, is the standard text on the subject from O’Reilly. It’s highly technical, with an emphasis on generating XSL-FO by using XSLT to transform XML documents.
  • XSL Formatting Objects Developer’s Handbook (2002), by Doug Lovell, is one of the first, and still one of the best, books to delve specifically into the XSL-FO markup format, instead of simply assuming all XSL-FO would be generated by XSLT transformations.
  • Definitive XSL-FO (2003), by G Ken Holman, is more of a reference book than a tutorial. It explains the W3C specification in detail, covering topics that other XSL-FO guides leave out.

XSL-FO Is No Legacy Tool

XSL-FO is one of several XML-related standards that haven’t gotten as much attention since HTML and XML diverged. However, it is an extremely useful technology, and there’s nothing “legacy” about it. It continues to be in heavy use in enterprise and industrial settings that need to manage content and data across multiple media and publications.

XPath

XPath is a little different and more general than XSLT and XSL-FO. It is a query language used to locate and select content in an XML or HTML document.

XPath is a more powerful alternative to CSS Selectors. While CSS Selectors are easier to use, but are not always feasible if the document author has omitted common attributes such as id and class. XPath provides a way to specify any node in a DOM tree, even without these attributes. This makes it well-suited for web scraping and document analysis.

The language got a lot of attention when it was introduced in 1999. It is still useful and thriving today, but there aren’t as many contemporary tutorials as there might be. So we’ve collected the best available resources for learning and using XPath.

XPath Tutorials

  • XPath Tutorial from W3Schools is multi-part, in-depth explanation of XPath, with lots of practical examples and a good explanation of how XPath is related to other XML standards.
  • XPath Overview from Tutorials Point provides a easy to follow introduction to the language.
  • XPath Tutorial is a community-written tutorial on XPath basics from the Edutech Wiki.
  • The 10-Minute XPath Tutorial is a Perl-focused introduction to XPath, primarily geared toward system administrators.
  • XML XPath Tutorial is a Java-based introduction to XPath.
  • XPath Syntax is a highly technical tutorial on XPath, with a focus on .NET implementation, from Microsoft Developer Network.
  • How XPath Works is a Java-focused introduction to XPath, from Oracle.
  • What Can XPath Do for Me? is an introduction to XPath, for the benefit of mostly non-tech-savvy academics working in the humanities. This is a very good place to start if you are using XPath to query documents for scholarly research.
  • XPath Tutorial is a gentle introduction to using XPath, with a focus on searching ebook content.
  • XPath Tutorial Application is an interesting meta-tutorial from Microsoft Developer Network. It helps you learn XPath by showing how to build an application which uses XPath to teach XPath.
  • Learning XPath by Example is a visual tutorial on the language.
  • XPath for Web Scraping explains how to use XPath to programmatically extract content from web pages with Python.
  • PHP Scraping Using DOM and XPath Tutorial explains how to use XPath and PHP to programmatically extract content from web pages.
  • Mozilla Developer Network has a wide array of XPath-related documentation, tutorials, reference materials, and tools.

XPath Reference

XPath Tools

  • Free Online XPath Tester is an online XPath tester that lets you test expressions against online document via URL.
  • XPath-Tools is a set of command-line utilities for extracting data from HTML and XML documents.
  • XPath Visualizer is a Windows desktop tool that provides a visual representation of an XML or HTML tree, and the results of XPath queries performed against it.
  • XMLSpy, an XML editor, has a built-in Xpath Editor and Debugger that provides an number of tools for working with XPath, including auto-completion, deep path suggestions, and multi-file evaluations.
  • Stylus Studio has several useful XPath tools, including a visual expression generator that will help you build an XPath query by selecting content within a document. Their XPath tutorials are also worth checking out.

XPath Libraries and Implementations

  • XPath module for the Gnome XML C Parser provides XPath support in C, and also has bindings to Python, Perl, C++, PHP, Pascal, Ruby, and Tcl.
  • XPath Library for the OCaml programming language.
  • XPath gem provides XPath support in Ruby.
  • Xpath npm package provides Xpath implementation and helpers for Node.js.
  • XPathJs is a pure JavaScript implementation of XPath, created by Google.

XPath Books

  • XPath 2.0 Programmer’s Reference (2004), by Michael Kay, is the definitive classic reference work on XPath.
  • Definitive XSLT and XPath (2001), by G Ken Holman, is the authoritative guide to XPath and XSLT. It is highly technical and also provides much of the philosophical and theoretical background to how XML is designed and what is actually contained in the specifications. There are easier books for learning how to use XPath, but few that will help you really understand it in this much depth.
  • Python and XML (2001), by Jones and Drake, includes sections on using Python to query and manipulate XML documents via XPath.
  • XPath Kick Start: Navigating XML with XPath 1.0 and 2.0 (2003), by Steven Holzner, is a concise book designed for beginners.
  • XPath and XPointer: Locating Content in XML Documents (2002), by John Simpson, is a relatively short book covering XPath basics. It’s speculation on the future of the standard, from its 2002 vantage point, is a little dated now, but the primary content is still highly relevant.
  • XSLT and XPath On The Edge (2001), by Jeni Tennison, is a cookbook style reference manual with tons of highly useful example queries.
  • Beginning XSLT and XPath: Transforming XML Documents and Data (2009), by Ian Williams, is a very good introduction to using XSLT and XPath. Written a few years later than most other popular books on the topic, this book has the benefit of several years of experience with the standard.

XPath Summary

XPath may not seem trendy right now. When it was first released, most people thought XML was going to become the standard language for web markup. But HTML 5 broke away from strict XML, and JSON has displaced XML as the dominant data serialization format.

However, XPath is as relevant as ever. It is still the most reliable way to query information in an XML (or HTML) document, and is the basis for XSLT. If you’re interested in web scraping, web search and indexing, or document analysis, XPath continues to be an important skill.

Summary

If you work with XML documents, you will undoubtedly run into situations where you need a way to translate them into another form. Thus, it is critical to know the tools we’ve discussed here.


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Text written by Adam Michael Wood. Compiled and edited by Frank Moraes.

About Adam Michael Wood

Adam specializes in developer documentation and tutorials. In addition to his writing here, he has authored engineering guides and other long-form technical manuals. Outside of work, Adam composes and performs liturgical music. He lives with his wife and children in California.