Supporting Works on Computers and Paper
Since the mid-1980s, the electronic publishing community has faced the issue of supporting works targeted for use on computers, or on computers and paper (i.e., for dual publishing). To achieve maximal flexibility, it is desirable to separate the description of document structure from the rendering of that structure into some paper or screen form. SGML and, later, XML, were devised with this goal in mind.
SGML and XML
SGML and XML are markup languages. In particular, they are meta-languages. One can provide a document type definition (DTD) that conforms to the rules of SGML, and then be able to create documents of that type. The same applies to XML. However, with XML, it is possible instead to use a schema to specify the type of document considered, or to just have markup without a DTD, demonstrating that this indeed is an eXtensible Markup Language.
Other Markup Languages
In the SGML/XML family are other markup schemes, or applications of these. HyTime extends SGML with architectural forms to allow handling of rich multimedia and hypermedia content. RDL, developed by e-Numerate Solutions, Inc., allows handling of numeric content in an application of XML, while XBRL applies XML to business reporting. For many ETDs, tailored markup schemes (e.g., MathML for mathematics, or chemical markup language), also are used for particular types of content expression.
The following subsections explain further how ETDs can be produced using markup languages.
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