XML (eXtensible Markup Language) is a text-based, semantic programming language. It enables information to be structured for easy exchange between platforms and systems without knowledge of specific hardware or software. That means the same XML file can be used and displayed on multiple platforms, as long as the file conforms to the rules of the XML document defining the type of information, known as a schema or Document Definition Type (DTD).
Anyone can define a schema or DTD for their own use; many large publishing houses find it useful to do so.
International standards bodies also define DTDs, so that a common format can be used for file creation and dissemination. For example, the IDPF defines the EPUB DTD, including its file- name extension of .epub, so that any valid .epub file can be used on any platform or device that supports EPUB. Other well-known XML DTDs are DITA for technical documents, DAISY for accessibility to vision-impaired readers, NLM for scientific, technical and medical journals, and SPL for product labelling in the pharmaceutical industry.
XML in one DTD can also be translated into another DTD, using a process called XSLT (Extensible Stylesheet Language Transformations).
The appearance of an XML file is controlled by a stylesheet that sets out the rules for display, so that the same XML file can be displayed differently using different stylesheets. In some cases, such as CSS for HTML and EPUB, there are international standards for stylesheets too.
Typefi Publish includes support for EPUB, NLM, DITA, SPL and Docbooks. With the aid of an XSLT, Typefi Publish also supports custom XML schemas. See XML Import/Export Modules for more information on specific DTDs supported by Typefi.
The main benefits are consistency, portability and “future-proofing”. XML applied according to a schema ensures a consistent structure is applied to similar types of content. Validation processes can also highlight when an important structural element is missing.
Used in conjunction with a stylesheet or template (as with Typefi Publish), XML also ensures that the same design is applied consistently to all components with the same XML tag.
Content saved as XML is highly portable, meaning it can be applied against different stylesheets, translated into different DTDs, and also archived in a format that will always be readable and usable by both humans and computers, so it will never be obsolete.