XML is the result of a long evolution of data packaging reaching back to the days of punched cards. It is useful to trace this path to see what mistakes and discoveries influenced the design decisions.
Early electronic formats were more concerned with describing how things should look (presentation) than with document structure and meaning. troff and TEX, two early formatting languages, did a fantastic job of formatting printed documents, but lacked any sense of structure. Consequently, documents were limited to being viewed on screen or printed as hard copies. You couldn't easily write programs to search for and siphon out information, cross-reference information electronically, or repurpose documents for different applications.
Generic coding, which uses descriptive tags rather than formatting codes, eventually solved this problem. The first organization to seriously explore this idea was the Graphic Communications Association (GCA). In the late 1960s, the GenCode project developed ways to encode different document types with generic tags and to assemble documents from multiple pieces.
The next major advance was Generalized Markup Language (GML), a project by IBM. GML's designers, Charles Goldfarb, Edward Mosher, and Raymond Lorie, intended it as a solution to the problem of encoding documents for use with multiple information subsystems. Documents coded in this markup language could be edited, formatted, and searched by different programs because of its content-based tags. IBM, a huge publisher of technical manuals, has made extensive use of GML, proving the viability of generic coding.
 Cute fact: the acronym GML also happens to be the initials of the three inventors.
Inspired by the success of GML, the American National Standards Institute (ANSI) Committee on Information Processing assembled a team, with Goldfarb as project leader, to develop a standard text-description language based upon GML. The GCA GenCode committee contributed their expertise as well. Throughout the late 1970s and early 1980s, the team published working drafts and eventually created a candidate for an industry standard (GCA 101-1983) called the Standard Generalized Markup Language (SGML). This was quickly adopted by both the U.S. Department of Defense and the U.S. Internal Revenue Service.
In the years that followed, SGML really began to take off. The International SGML Users' Group started meeting in the United Kingdom in 1985. Together with the GCA, they spread the gospel of SGML around Europe and North America. Extending SGML into broader realms, the Electronic Manuscript Project of the Association of American Publishers (AAP) fostered the use of SGML to encode general-purpose documents such as books and journals. The U.S. Department of Defense developed applications for SGML in its Computer-Aided Acquisition and Logistic Support (CALS) group, including a popular table formatting document type called CALS Tables. And then, capping off this successful start, the International Standards Organization (ISO) ratified a standard for SGML (ISO 8879:1986).
SGML was designed to be a flexible and all-encompassing coding scheme. Like XML, it is basically a toolkit for developing specialized markup languages. But SGML is much bigger than XML, with a more flexible syntax and lots of esoteric parameters. It's so flexible that software built to process it is complex and generally expensive, and its usefulness is limited to large organizations that can afford both the software and the cost of maintaining SGML environments.
The public revolution in generic coding came about in the early 1990s, when Hypertext Markup Language (HTML) was developed by Tim Berners-Lee and Anders Berglund, employees of the European particle physics lab CERN. CERN had been involved in the SGML effort since the early 1980s, when Berglund developed a publishing system to test SGML. Berners-Lee and Berglund created an SGML document type for hypertext documents that was compact and efficient. It was easy to write software for this markup language, and even easier to encode documents. HTML escaped from the lab and went on to take over the world.
However, HTML was in some ways a step backward. To achieve the simplicity necessary to be truly useful, some principles of generic coding had to be sacrificed. For example, one document type was used for all purposes, forcing people to overload tags rather than define specific-purpose tags. Second, many of the tags are purely presentational. The simplistic structure made it hard to tell where one section began and another ended. Many HTML-encoded documents today are so reliant on pure formatting that they can't be easily repurposed. Nevertheless, HTML was a brilliant step for the Web and a giant leap for markup languages, because it got the world interested in electronic documentation and linking.
To return to the ideals of generic coding, some people tried to adapt SGML for the Webor rather, to adapt the Web to SGML. This proved too difficult. SGML was too big to squeeze into a little web browser. A smaller language that still retained the generality of SGML was required, and thus was born the Extensible Markup Language (XML).
Dissatisfied with the existing formats, a group of companies and organizations began work in the mid-1990s at the World Wide Web Consortium (W3C) on a markup language that combined the flexibility of SGML with the simplicity of HTML. Their philosophy in creating XML is embodied by several important tenets:
In other words, markup languages need to fit their data snugly. Rather than invent a single, generic language to cover all document types (badly), let there be many languages, each specific to its data. Users can choose element names and decide how they should be arranged in a document. The result will better labeling of data, richer formatting possibilities, and enhanced searching capability.
A document should be marked up in such a way that there is only one way to interpret the names, order, and hierarchy of the elements. Consider this example from old-style HTML:
<html> <body> <p>Here is a paragraph. <p>And here is another. </body> </html>
Before XML, this was acceptable markup. Every browser knows that the beginning of a <p> signals the end of an open p element preceding it as well as the beginning of a new p element. This prior knowledge about a markup language is something we don't have in XML, where the number of possible elements is infinite. Therefore, it's an ambiguous situation. Look at this example; does the first element contain the other, or are they adjacent?
<flooby>an element <flooby>another element
You can't possibly know, and neither can an XML parser. It could guess, but it might guess incorrectly. That's why XML rules about syntax are so strict. It reduces errors by making it more obvious when a document has mis-coded markup. It also reduces the complexity of software, since programs won't have to make an educated guess or try to fix syntax mistakes to recover. It may make it harder to write XML, since the user has to pay attention to details, but this is a small price to pay for robust performance.
For your document to have maximum flexibility for output format, you should strive to keep the style information out of the document and stored externally. Documents that rely on stylistic markup are difficult to repurpose or convert into new forms. For example, imagine a document that contains foreign phrases that are marked up to be italic, and emphatic phrases marked up the same way, like this:
<example>Goethe once said, <i>Lieben ist wie Sauerkraut</i>. I <i>really</i> agree with that statement.</example>
Now, if you wanted to make all emphatic phrases bold but leave foreign phrases italic, you'd have to manually change all the <i> tags that represent emphatic text. A better idea is to tag things based on their meaning, like this:
<example>Goethe once said, <foreignphrase>Lieben ist wie Sauerkraut</foreignphrase>. I <emphasis>really</emphasis> agree with that statement.</example>
Instead of being incorporated in the tag, the style information is defined in another place, a document called a stylesheet. Stylesheets map appearance settings to elements, acting as look-up tables for a formatting program. They make things much easier for you. You can tinker with the presentation in one place rather than doing a global search and replace operation in the XML. If you don't like one stylesheet, you can swap it for another. And you can use the same stylesheet for multiple documents.
Keeping style out of the document enhances your presentation possibilities, since you are not tied to a single style vocabulary. Because you can apply any number of stylesheets to your document, you can create different versions on the fly. The same document can be viewed on a desktop computer, printed, viewed on a handheld device, or even read aloud by a speech synthesizer, and you never have to touch the original document sourcesimply apply a different stylesheet. (It is of course possible to create presentation vocabularies in XMLXSL-FO is an excellent example. In XSL-FO's case, however, its creators expect developers to create XSL-FO through XSLT stylesheets, not directly.)
For XML to gain widespread acceptance, it had to be simple. People don't want to learn a complicated system just to author a document. XML 1.0 is intuitive, easy to read, and elegant. It allows you to devise your own markup language that conforms to some logical rules. It's a narrow subset of SGML, throwing out a lot of stuff that most people don't need.
Simplicity also benefits application development. If it's easy to write programs that process XML files, there will more and cheaper programs available to the public. XML's rules are strict, but they make the burden of parsing and processing files more predictable and therefore much easier.
Some markup languages are so lenient about syntax that errors go undiscovered. When errors build up in a file, it no longer behaves the way you want it to: its appearance in a browser is unpredictable, information may be lost, and programs may act strangely and possibly crash when trying to open the file.
The XML specification says that a file is not well-formed unless it meets a set of minimum syntax requirements. Your XML parser is a faithful guard dog, keeping out errors that will affect your document. It checks the spelling of element names, makes sure the boundaries are airtight, tells you when an object is out of place, and reports broken links. You may carp about the strictness, and perhaps struggle to bring your document up to standard, but it will be worth it when you're done. The document's durability and usefulness will be assured.
There's no good reason to confine markup in a narrow cultural space such as the Latin alphabet and English language. And yet, earlier markup languages do just that. Irked by this limitation, XML's designers selected Unicode as the character set, opening it up to thousands of letters, ideographs, and symbols.