Learning Java, 4th Edition

A Bit of Background

XML and HTML are called markup languages because of the way they add structure to plain-text documents—by surrounding parts of the text with tags that indicate structure or meaning, much as someone with a pen might highlight a sentence and add a note. While HTML predefines a set of tags and their structure, XML is a blank slate in which the author gets to define the tags, the rules, and their meanings.

Both XML and HTML owe their lineage to Standard Generalized Markup Language (SGML)—the mother of all markup languages. SGML has been used in the publishing industry for decades (including at O’Reilly). But it wasn’t until the Web captured the world that it came into the mainstream through HTML. HTML started as a very small application of SGML, and if HTML has done anything at all, it has proven that simplicity reigns.

Text Versus Binary

When Tim Berners-Lee began postulating the Web back at CERN in the late 1980s, he wanted to organize project information using hypertext with links embedded in plain text.^[49] When the Web needed a protocol, HTTP—a simple, text-based client-server protocol—was invented. So, what exactly is so enchanting about the idea of plain text? Why, for example, didn’t Tim turn to the Microsoft Word format as the basis for web documents? Surely a binary, non-human-readable format and a similarly machine-oriented protocol would be more efficient? Since the Web’s inception, there have now been literally trillions of HTTP transactions. Was it ...