XSLT, 2nd Edition

XML’s Heritage

XML’s heritage is in the Standard Generalized Markup Language (SGML). Created by Dr. Charles Goldfarb in the 1970s, SGML is widely used in high-end publishing systems. Unfortunately, SGML’s perceived complexity prevented its widespread adoption across the industry (SGML also stands for “sounds great, maybe later”). SGML got a boost when Tim Berners-Lee based HTML on SGML. Overnight, the whole computing industry was using a markup language to build documents and applications.

The problem with HTML is that its tags were designed for the interaction between humans and machines. When the Web was invented in the late 1980s, that was just fine. As the Web moved into all aspects of our lives, HTML was asked to do lots of strange things. We’ve all built HTML pages with awkward table structures, 1-pixel GIFs, and other nonsense just to get the page to look right in the browser. XML is designed to get us out of this rut and back into the world of structured documents.

Whatever its limitations, HTML is the most popular markup language ever created. Given its popularity, why do we need XML? Consider this extremely informative HTML element:

<td>12304</td>

What does this fascinating piece of content represent?

• Is it the postal code for Schenectady, New York?

• Is it the number of light bulbs replaced each month in Las Vegas?

• Is it the number of Volkswagens sold in Hong Kong last year?

• Is it the number of tons of steel in the Sydney Harbour Bridge?

The answer: maybe, maybe not. The point of ...