Foundational Standards and Specifications Used in Markup Language Technologies
Despite the convenience of a label, there is no principled grounds for the characterization of these eight (8) particular standards as "core standards" related to markup language technologies. Depending upon the analytical perspective (breadth of use, location in the Internet stack, natural competitors, etc.) a wide range of standards could be considered "core," "critical," or "foundational" to the Internet technology infrastructure.
These eight selections represent some of the more common standards domains considered interesting and/or important for developing markup-based Internet applications. For the beginner, they would be natural candidates for an introduction -- enabling one to do something useful with markup. Historically, SGML and XML are the chief metalanguage standards used to define specific markup languages. The foundational principle of both metalanguages is to separate the specification for structure/vocabulary/serialization from other specifications that define processing of the 'document/data' instance. For visual presentation, then, something like XSL, CSS, or DSSSL is necessary to define the processing semantics. Of course, visual display is just one of many kinds of data processing applicable to XML-encoded information. Many other important standards underlie both SGML and XML (e.g., standards for character encodings) and many adjunct, collateral standards are needed in connection with both XML and SGML to make them useful. Data structured using markup does nothing: it just sits there. For processing, one may need standardized formal models for the information set (e.g., XML Infoset) and for interfaces (e.g., DOM).
XML. The Extensible Markup Language is a profile or restricted subset of SGML as a metalanguage. XML is used to define application-specific XML vocabularies, or XML markup languages.
SGML. The Standard Generalized Markup Language (ISO 8879:1986) is historically the dominant (meta-)markup language.
Schemas. Whereas SGML and XML 1.0 use a special-syntax notation to formally define markup language grammars (DTDs), XML applications are using a wide range of formalisms to express datatypes and other (semantic) constraints.
XSL/XSLT/XPath. The Extensible Stylesheet Language is a language for expressing 'style'(sheets). Its components include XSL Transformations (XSLT), an XML Path Language (XPath), and XSL Formatting Objects (XSL-FO, an XML vocabulary for specifying formatting semantics).
XLink/XPointer. XLink (XML Linking Language) supplies basic facilities for defining links between resources. The XML Pointer Language (XPointer) is partitioned into four parts; it supports addressing into the internal structures of XML documents.
XML Query. Various query languages have been proposed and implemented for querying XML documents.
CSS. W3C's Cascading Style Sheets provide a simple mechanism for adding style (e.g., fonts, colors, spacing) to Web documents.
SVG. Scalable Vector Graphics is a language for describing two-dimensional graphics in XML. Other graphics formats can be used in XML documents, of course.