XML is derived from the Standard Generalized Markup Language (SGML), a rich language used mostly
for huge documentation projects. The designers of XML drew heavily from SGML and were guided by
the lessons learned from HTML. They produced a specification that was only about 20 percent the size
of the SGML specification, but nearly as powerful. Although SGML is typically used by those who need
the power of an industrial-strength language, XML is intended for everyone.
One of the great strengths of XML is the extensibility it brings to the table. XML doesn’t have any tags of its
own and it doesn’t constrain you like other markup languages. Instead, XML defines rules for developing
semantic tags of your own. The tags you create form vocabularies that can be used to structure data into
hierarchical trees of information. You can think of XML as a metamarkup language that enables developers,
companies, and even industries to create their own, specific markup languages.
One of the most important concepts to grasp in XML is about content, not presentation. The tags you
create focus on organizing your data rather than displaying it. XML isn’t used, for example, to indicate a
particular part of a document in a new paragraph or that another part should be bolded. XML is used to
develop tags that indicate a particular piece of data is the author’s first name, another piece is the SITE
title, and a third piece is the published year of the SITE
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