XML, like HTML, comes from standard generalized Markup Language, the standard universal Markup Language, or SGML. SGML existed long before the web was invented. SGML, as its name puts it, is a common language that describes documents with tags, including a series of document type definitions (DTD), where the meaning of markup is defined in the DTD, so SGML's syntax can be extended. SGML is so large that it is neither easy to learn nor easy to use, and it is difficult to implement on the computer. In view of these factors, researchers at CERN, the European Center for Nuclear Physics, proposed an HTML language based on the ability of computer technology at the time (1989).
HTML uses only a small portion of the markup in SGML, such as HTML 3.2, which defines 70 kinds of tags. For ease of implementation on the computer, the HTML-defined markup is fixed, that is, HTML syntax is not extensible, and it does not need to contain DTDs. This fixed syntax for HTML makes it easy to learn, and it's easy to develop HTML browsers on your computer. It is because of the simplicity of the HTML, so that Web technology from the computer industry to the whole society, to thousands of households, the development of the web is at its zenith.
In recent years, with the application of the Web more and more extensive and in-depth, people gradually feel that the HTML is not enough, HTML too simple syntax seriously hinder the use of it to perform complex forms. Despite the introduction of a new version of HTML, there are scripts, tables, frames, and so on, but never meet the growing demand. On the other hand, in recent years the development of computer technology is also very rapid, can be achieved than the original creation of HTML more complex web browsers, so the development of a new Web page language is both necessary and possible.
It is recommended that SGML be used directly as a Web language, which can solve the difficulties of HTML. But SGML is too big for users to learn and use, not to mention, to fully implement SGML browser is very difficult, so naturally think of using only a subset of SGML, so that the new language is easy to use and easy to implement. It is in this context that the Web Standardization Organization's consortium recommends the use of a streamlined SGML version of--xml.
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