The Gist of XML
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Posted on Friday, 23rd October 2015 by admin	
Technically, XML is not actually a Web technology, but, clearly, it has a close association with the Web. After all, it was developed by the W3C, and one of the first things they didi with it was to develop XHTML, so the relationship is pretty close.
Today, XML is used as a popular way of storing and exchanging data. It is at the heart of many document formats, as well as configurations formats. This article looks at some of the background of XML.

Some History
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HTML (HyperText Markup Language) was based on SGML (Standard Generalized Markup Language). SGML is not so much a language as a meta language: it is actually a set of rules for defining your own markup language. It doesn’t have its own vocabulary, but describes how you would create your own vocabulary, and how you would structure the different parts of your document.

SGML relies on a Document Type Definition (DTD) to actually define the vocabulary and structure: the DTD describes the elements which can be used and where they belong in your document. Of course, it cannot actually define the purpose of the element, but, hopefully the vocabulary you create will make that clear, along with any document you might provide.

SGML is a general-purpose base from which other languages might be derived. Foe Example, the OED uses its own SGML-based language for its dictionary. The most famous derived language is, of course HTML.