Procedural and Generalized

The vast majority of Web docu-ments are created and presentedin the HyperText Markup Lan-guage (HTML). HTML is well suited for hypertext linking and the display of small, relatively simple documents. HTML is an application of the Standard Generalized Markup Language (SGML), defined in ISO 8879:1986. SGML is a metalanguage designed to define docu-ment formats. SGML allows documents to describe their own grammar, which is implemented as a set of tags and the structural relationship that these tags represent. Although HTML also pro-vides for a tag set that makes it easy to build Web documents, the HTML tag set is comparatively small and cannot be extended. This lack of flexibility limits HTML’s ability to address extensibility, structure, and validation [1]: • Extensibility: HTML does not allow users to specify their own tags or attributes, which limits their ability to parameterize or semantically qualify their data. • Structure: HTML does not support the specification of deep structures needed to represent database schemas or object-oriented hierarchies. • Validation: HTML does not sup-port the kind of language specifica-tion that allows consuming applica-tions to check data for structural validity. At the other end of the functionality spectrum is SGML. SGML allows for the extensibility, structure, and valida-tion that are missing in HTML. With SGML, document formats can be de-fined, and extremely large document repositories can be managed. However, SGML implementations are expensive, and SGML provides many features that are either unnecessary for Web publish-ing or require a large effort to implement in a Web environment. Enter the Extensible Markup Lan-guage, (XML). As reported by Tim