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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 our NIMAS Exemplars, appears below:

XML (EXtensible Markup Language)

Universal format for structured documents and data. Set of rules, guidelines, and conventions for designing text formats for data.

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Any identifiable object within a document.

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Descriptive markup component delimiting the start or end of an element.

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HTML (HyperText Markup Language)

Non-proprietary markup language based on SGML. Created and processed by a wide range of tools from simple to complex.

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Value or descriptive information associated with an HTML, XHTML, or XML element.

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Equipment or system where principal function is creation, conversion, duplication, control, display, interchange, transmission, reception, or broadcast of data.

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PDF (Portable Document Format)

Universal computer file type used to exchange and view documents on any computer with Adobe Acrobat or Foxit Reader software installed.

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National Instructional Materials Accessibility Standard (NIMAS)

A technical standard used to produce XML-based source files for print-based educational materials.

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<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>
<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>

See all of this exemplar, as well as others, in our NIMAS Exemplars section.

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.

This page is intended as a "resource for users, authors, developers, and the interested reader." 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.

W3C Recommendation
The World Wide Web Consortium's XML Recommendation (2nd edition, 2006) provides exhaustive XML standards content from some of those who developed it. Relatively technical in nature, it provides specific requirements information regarding XML standards. is a huge website geared toward XML and standards. Its About page states its purpose as "for those interested and involved in XML-related standards and specifications." describes a primary goal of 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 currently succeeds as the XML industry's standards information clearinghouse.

See the Creating NIMAS Files page for more information about NIMAS mark-up and the development of NIMAS-conformant files and the rest of the Best Practices for Publishers & Software Developers section for information about production/tools and the development of accessible content.

W3C (World Wide Web Consortium)

Promotes evolution and ensures interoperability of the World Wide Web, producing specifications and reference software for free use around the world.

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NIMAS-conformant files

XML files valid to the NIMAS technical specification used to create accessible specialized formats of print-based instructional materials.

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Content, activity, or technology that is usable by everyone with equivalent ease of use.

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