Understanding XML

XML is a mark-up language that codes text and other information for a variety of uses. XML stands for eXtensible Mark-up Language. XML is made up of a series of elements, most of which have two tags. These tags go around pieces of content, say, text or numbers, with one tag of the element at the start of a piece of content and the other tag at its end. In this way, XML is similar to HTML. To create an XML file, a piece of content is coded with XML elements and saved as an -.xml document. XML elements can also have other components to make the use of them more versatile and complex (for example, attributes and their values). XML also includes entities, which stand for specific characters, such as the carets used in XML tags.

XML is particularly suited to content intended for publication, for example, content that prior to XML technology would be print-based (books, articles, manuals); yet XML is also appropriate for use with databases, web sites, etc. XML allows different systems to communicate through the use of it. A major benefit of XML is its ability to provide for single-source publishing. An XML file serves as a source file, and, from it, many different output files (versions or formats) can be produced. An example of this would be using a single XML file to create outputs in Word, PDF, and HTML.

An important characteristic of XML is the fact that it allows content to be prepared entirely separate from formatting or other content manipulations. This separation permits the greatest number of outputs with the least amount of work and enables corrections, changes, new editions, updates, and so on to be made easily and efficiently. It is also readable in its proper form—being largely self-explanatory, especially in context, means that a reader, user, and interpreter of an XML document need not be its author. XML is also highly valuable because it is not tied to a single software application or operating system, making it free of application and computer upgrades; it is widely used and accepted internationally; and users can create XML tags if necessary, thereby making XML flexible, adaptable, and unlikely to become obsolete.

A snippet of NIMAS-conformant XML code, taken from one of CAST's exemplars, appears below:

<level1 id="L001" class="chapter">
<h1 id="L001.H01" class="chapter">Chapter 24: The Great Depression</h1>
<pagenum id="page_1" page="normal">1</pagenum>
<level2 id="L001.001" class="mainsection">
<h2 id="L001.001.H01" class="mainsection">Overview</h2>
<p id="L001.001.P001">During the 1920s, the United States saw a time of great prosperity. However, that would all change with the stock market crash of 1929. The country and the world would be plunged into an economic and social depression.</p>
<p id="L001.001.P002">Companies were going bankrupt, banks were shutting down, and unemployment was skyrocketing.</p>
<imggroup>
<img id="L001.001.P002-001" src="./images/p002-001.jpg" alt="Black and white photo of a makeshift home during the Depression"/>
<prodnote id="L001.001.longdesc002-001" imgref="L001.001.P002-001"><p>This black and white photo shows a makeshift home during the Depression. Two clotheslines are strung across the center of the picture in front of a wooden shed. Three children play on the ground in front of the shed and beneath the clothes hanging from the clotheslines. A small table with a large wooden bucket can be seen on the left. The ground is dirt or sand and is littered with scraps of wood and metal, baskets, and other debris. A leafless tree can be seen in the background on the left.</p></prodnote>
<caption id="L001.001.C01" imgref="L001.001.P002-001">The Great Depression caused immediate hardship on everyday life. Hundreds of thousands of people lost their homes, their jobs, and their dignity. Families, like the one shown above, were forced to live in make-shift camps that were overcrowded and unsanitary.</caption>
</imggroup>
</level2>
</level1>

To see all of this exemplar, as well as others, go to CAST's NIMAS Exemplars page.

Wikipedia's XML page
This page contains information on what XML is, its history, features, requirements, extensions, and more. It is written without an assumption of knowledge base. It also lists several good introductory tutorials.

The XML FAQ
This page is intended as a "resource for users, authors, developers, and the interested reader."¹ It includes basic as well as more detailed information for those who are progressing to using XML in their work. This FAQ list is quite popular in the XML community and is frequently cited as a resource on a variety of web sites.

XML Glossary
This XML glossary, provided by the German company Software AG, is a useful alphabetical listing of XML and related information. It would serve as a good starting point for browsing for knowledge and understanding, as well as a way to find definitions and explanations of software, terms, organizations, etc.

W3C Recommendation
The World Wide Web Consortium's XML Recommendation (2nd edition, 2004) provides exhaustive XML standards content from some of those who developed it. Relatively technical in nature, it provides specific requirements information regarding XML standards.

XML.org
XML.org is a huge web site geared toward XML and standards. Its About page states its purpose as "for those interested and involved in XML-related standards and specifications." Service-architecture.com describes a primary goal of XML.org as the effort "to minimize overlap and duplication in XML languages and XML standard initiatives by providing public access to XML information and XML schemas"—a point worth noting, as XML.org currently succeeds as the XML industry's standards information clearinghouse.

Practical XML
From the introduction: "a presentation on the overview of and practical uses for XML. Various XML standards and tools are covered, including XSLT, a language for transforming XML. Two real-world uses of XML are surveyed…." (provided copyright-free by Jeremy Mates).

See the Creating NIMAS Files document for more information about NIMAS mark-up and the development of NIMAS-conformant files and the Content Development and Design page for information about production/tools and the development of accessible content.

1 - Flynn, P. (2006). The XML FAQ. http://xml.silmaril.ie/.