HTML Unleashed. SGML and the HTML DTD: Introduction

GML is the substratum on which HTML was conceived and, therefore, is responsible for many of HTML's strengths and weaknesses.  SGML stands for Standard Generalized Markup Language; this is a formal system designed for building text markup languages.  It is not a markup system by itself, however; think of it as a programming language to build working programs (HTML being one of them) rather than a program by itself.

In this chapter, you'll learn the foundations of SGML to see how (and why) HTML was built on top of it.  It's very instructive and engaging to trace the roots of the language and explore the conceptions of its creators.  In fact, you can't say you know HTML unless you're at least sketchily acquainted with its SGML heritage.

We'll analyze the definition of HTML in terms of SGML, consisting of SGML Declaration and document type definition, both for HTML version 4.0.  You'll see what valuable information can be elicited from these formal constructs and how they can be used for authoritative reference on HTML topics.  You'll also learn why an HTML document should conform to a DTD and how to ensure this using a validation service.

Knowledge of basic SGML concepts and syntax will provide you with a solid foundation for mastering HTML and will help you understand some of the peculiarities of the language.  The goal of this chapter, however, is not to teach you SGML or how to write SGML applications, but to show you how understanding SGML may aid in learning and applying HTML.