Where XML Came From

Document markup languages have been around for decades. In 1973 IBM developed the generalized markup language (GML). This was a markup language that was inserted into documents, primarily for formatting. However, it was a proprietary markup language.

Eight years of development, with input from hundreds of people in the document management community, led to the standard generalized markup language (SGML) in 1985. SGML was a comprehensive and widely adopted way to annotate documents with structure.

However, SGML was complex. It was complex to learn, and writing parsers and interpreters was also complex. In 1991 Tim Berners-Lee developed HTML as an application of SGML, tailored specifically for displaying text and graphics and supporting interdocument links. Due at least in part to its great simplicity, HTML and the World Wide Web took off.

HTML is a tagged markup language, but there is a fixed set of tags. The tags don't represent semantic concepts; the tags are layout and presentation clues. The tags that indicate "bold face" or "table" do not determine what the bold-faced item is, nor whether the items in the rows of the table have any relationship to the column headings.

By 1996 several people had come to the conclusion that the World Wide Web would eventually outgrow HTML. The Web needed the expressiveness of SGML without its complexity, and XML was created out of that vision. The team that authored the XML specification did so with the goal to get 80% of the expressiveness out of 20% of the grammar. The result is that XML is a derivative and a subset of SGML.

What we have in XML is a simpler and more consistent language, with a high degree of extensibility. XML is stricter than HTML about grammatical elements such as tag matching. The original spec was 28 pages long, and even after years of debate and revision it is only 38 pages long.