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The following sections introduce each of these markup concepts. Delimited by angle brackets, most elements identify the nature of the content they surround.
Some elements may be empty, as seen above, in which case they have no content.
These include vector graphics, e-commerce transactions, mathematical equations, object meta-data, server APIs, and a thousand other kinds of structured information. In HTML, both the tag semantics and the tag set are fixed. Defining XML as an application profile of SGML means that any fully conformant SGML system will be able to read XML documents.
However, using and understanding XML documents does not require a system that is capable of understanding the full generality of SGML.
Full SGML systems solve large, complex problems that justify their expense.
Viewing structured documents sent over the web rarely carries such justification.
The XML specification defines a standard way to add markup to documents.
When finished, these will bring namespaces, linking, schemas, stylesheets, and more to the table.
This introduction to XML presents the Extensible Markup Language at a reasonably technical level for anyone interested in learning more about structured documents.
As time goes on, additional requirements will be addressed by other specifications.
Currently (Sep, 1998), namespaces (dealing with tags from multiple tag sets), a query language (finding out what's in a document or a collection of documents), and a schema language (describing the relationships between tags, DTDs in XML) are all being actively pursued.Please consult the discussion of EBNF in the appendix of this article for a detailed description of how this grammar works. A Simple XML Document XML documents are composed of markup and content.