A brief digression into other DTDs may help make clear what
parts of the previous section were specific to DocBook and what
parts are general to all structural-markup languages. TEI (Text
Encoding Initiative) is a large, elaborate DTD used primarily in
academia for computer transcription of literary texts. TEI's
Unix-based toolchains use many of the same tools that are
involved with DocBook, but with different stylesheets and (of
course) a different DTD. XHTML, the latest version of HTML, is
also an XML application described by a DTD, which explains the
family resemblance between XHTML and DocBook tags. The XHTML
toolchain consists of web browsers and a number of ad-hoc HTML-
to-print utilities. Many other XML DTDs are maintained to help
people exchange structured information in fields as diverse as
bioinformatics and banking. You can look at a list of
repositories to get some idea of the variety out there.