HTML & XHTML: The Definitive Guide, 6th Edition

Languages and Metalanguages

A language is composed of commonly accepted symbols that we assemble in a meaningful way in order to express ourselves and to pass along information that is intelligible to others. For example, English is a language with rules (grammar) that define how to put its symbols (words) together to form sentences, paragraphs, and, ultimately, books like the one you are holding. If you know the words and understand the grammar, you can read the book, even if you don’t necessarily understand its contents.

An important difference between human and computer-based languages is that human languages are self-describing. We use English sentences and paragraphs to define how to create correct English sentences and paragraphs. Our brains are marvelous machines that have no problem understanding that you can use a language to describe itself. However, computer languages are not so rich and computers are not so bright that you could easily define a computer language with itself. Instead, we define one language—a metalanguage—that defines the rules and symbols for other computer languages.

Software developers create the metalanguage rules and then define one or more languages based on those rules.^[*] The metalanguage also guides developers who create the automated agents that display or otherwise process the contents of documents that use its language(s).

XML is the metalanguage the W3C created and that developers use to define markup languages such as XHTML. Browser developers ...