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Perl CGl Programming: No experience required.
Describing a DocumentSGML is by its name a generalized markup language, which means that it is used to define other markup languages. HTML, of course, is one of them. But you could use the SGML specification to define any descriptive language you like. The point is that no matter what language you come up with, the framework of SGML will ensure that it works in a device-independent way. Other people wont have any trouble reading your SGML-derived document with their SGML software. Markup languages are usually divided into two types:
There are so many procedural markup languages in use that it would be ridiculous to attempt to list them here. Every typesetting system has its own proprietary set of instructions. What distinguishes all of these procedural languages, however, is their specificity. In other words, the language requires you to order precisely what you want. For example, the change command is a feature of many typesetting systems. A typical cc order might look like this: $cc,12p3,helv,10,9.5 which sets the type column width to 12 picas, 3 points, the font to Helvetica, using 10-point characters on a 9.5-point set width. This is pretty specific.
The descriptive markup languages describe the formatting rather than ordering it. In using a descriptive language, you place commands in the document that give a general framework for how it should look. But the descriptive languages, SGML especially, actually discourage the use of a particular font, type size, or column measure. We can look to HTML for a good example. You can specify typefaces and sizes in an HTML document, but only in a very general way. The reason for this apparent restriction is simple when you think about the whole world of Web browsers out there: Who knows whether browser A or B will have the faintest idea what 10-point Helvetica characters are?
SGML was an obvious choice for creating a language for the World Wide Web, especially in the Webs early days when most browsers were text-based. Something very general was needed to get the Web to work on a wide variety of computers. HTML was the result. Defining Document TypesSGML is a metalanguage that can be used to define other descriptive markup languages to format documents and control how they are displayed or printed. The formal definition of an SGML-derived language is done in a documenta program, actuallycalled a document type definition or DTD. You will encounter this acronym a lot if you spend any time studying HTML. There are now three formal HTML DTDs in more-or-less widespread use on the World Wide Web:
The IETF Level 2 DTD is considered standard HTML; it is doubtful that there is a browser in existence that doesnt support it. Level 2 is descended directly from the Level 1 DTD developed in 1990 by Tim Berners-Lee at CERN, the birthplace of the World Wide Web. This DTD is the formal specification for HTML 2.0. The Level 3 DTD is intended to be the next HTML version: 3.2. It provides many useful extensions to the 2.0 standard, such as improved table formatting, inline figures within text, and mathematical notation commands. However, this DTD is the work of a committee, albeit an august one, consisting as it does of the members of the IETF HTML group and the World Wide Web Consortium (W3C). As a result, the specification is subject to change from time to time. Browser developers have to keep abreast of the changes if they wish to support Level 3.
Netscapes Mozilla DTD provides extensions to both the Level 2 and 3 HTML DTDs. The extensions were intended to enhance Netscapes Navigator browser, of course, but some of them, frames especially, proved to be so useful that other browser manufacturers began supporting them, too, making Mozilla a kind of de facto Internet standard. Microsoft decided early on to include the Mozilla features in its Internet Explorer browser. Thus, the two browsers that probably are in the most widespread use on the WebNavigator and IEboth adhere to the Mozilla DTD.
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