Understanding the Background of Web Page Tool Language XML (1)


I. The Background of XML Generation
Like HTML, XML comes from Standard Generalized Markup Language, or SGML. SGML existed long before the Web was invented. As its name suggests, SGML is a general language for describing document data with tags. It contains a series of document type definitions (DTD) which define the meaning of tags. Therefore, SGML’s grammar can be extended. SGML is very large. It is neither easy to learn nor easy to use. It is also very difficult to realize on computer. In view of these factors, researchers at the European Center for Nuclear Physics, the inventor of the Web, proposed the HTML language based on the computer technology capabilities of the time (1989).
HTML uses only a small portion of SGML tags, such as HTML 3.2, which defines 70 tags. In order to be easily implemented on a computer, the tags specified in HTML are fixed, that is, the HTML grammar is not extensible, and it does not need to contain DTD. The fixed grammar of HTML makes it easy to learn and use, and it is also very easy to develop HTML browsers on computers. It is because of the simplicity of HTML that Web technology moves from the computer world to the whole society and to millions of households. The development of Web is in full swing.
In recent years, with the application of the Web becoming more and more extensive and in-depth, people gradually feel that HTML is not enough, HTML is too simple grammar seriously hinders its use to express complex forms. Although HTML has introduced new versions one after another, and already has scripts, tables, frames and other expression functions, it still can not meet the growing demand. On the other hand, the development of computer technology has been very rapid in recent years. It has been possible to develop a new Web page language, which is much more complex than the original Web browser when HTML was invented.
Some people suggest using SGML as the Web language directly, which can certainly solve the difficulties HTML encounters. However, SGML is too large for users to learn and use. It is very difficult to fully implement the browser of SGML. It is natural to think of using only a subset of SGML to make the new language easy to use and implement. It is in this situation that Web Standardization Organization W3C proposes to use a streamlined version of SGML – XML.