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Document Type Definitions
Every SGML document must refer to the DTD for its document type. DTDs can appear very different from one another; you can structure and build them in many ways. DTDs are also very flexible.
Note:
Elements are structural components of a document that are surrounded by tags to tell an SGML system where those components begin and end and where to place them in the document. Attributes are further definitions for elements.
Listing 1.1 is the official Boeing DTD used by aircraft manufacturers for service bulletins. It’s large, but you can learn a lot from it. It has the following components:
- • Introductory comments and description (optional, but recommended)
- • Revision history (optional)
- • Opening delimiter for all declarations
- • Comment on how to refer to this public DTD
- • Notation callouts
- • Entity declarations
- • Revision tracking
- • Transmittal information
- • Front matter elements
- • Body sections and subsections
- • Appendix elements
- • Table elements
- • Graphics elements
- • List elements
- • Warning, caution, and note elements
- • Paragraph and reference elements
- • Elements for miscellaneous structures
- • Elements for effectivity coding
- • Close delimiter as a comment
- • Revision history (optional)
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Listing 1.1
The Boeing ATA Service Bulletin DTD
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This DTD is well documented with many comments to help you follow its logic.
Note:
For now, think of SGML as an enormous cookbook. The document content is the list of ingredients for a recipe. The DTD is the instructions for making the recipe. The SGML markup in the document combines the ingredients with the directions for a complete recipe of how to make a document.
SGML and the Internet
The DTD is how SGML maintains the integrity of the content and structure of a document. Its ability to do this explains, in part, why SGML is becoming so popular on the Internet.
The World Wide Web is written largely in HTML, which is one application of SGML. The explosive growth on the World Wide Web demands richer and larger sets of markup features in HTML. Newer revisions of HTML include greater numbers of SGML features. As the Web continues to grow, SGML will likely become the standard for more markup languages on the Internet.
How HTML Is Evolving To Include Larger Numbers of SGML Features
Each version of HTML is made from an SGML DTD in which all the HTML document features are defined. HTML tries to remain simpler than SGML, but as it evolves, it includes more SGML features.
You don’t create DTDs for HTML documents, because HTML is itself a DTD. Each version of HTML has its own DTD. HTML versions 1.0, 2.0, or 3.0 are actually three different DTDs, all of which are SGML applications.
HTML version 1.0 is a simple subset of SGML. It lacks flexibility and cannot handle tables, equations, and forms used to fill in information. Version 2.0 adds elegance and stability, but it still has difficulties. Version 3.0 is a proposed improvement upon version 2.0. Features that version 3.0 adds to HTML include:
- • Overriding stylesheets
- • Object linking and embedding
- • Multimedia and metafile extensions
- • Enhanced math and equation capabilities
- • Object linking and embedding
The Internet and SGML Browsing and Authoring Tools
SGML fans are fond of the Internet. Although HTML is currently the favorite markup language on the World Wide Web, it is moving toward the larger feature set of SGML. Consequently, more SGML tools are starting to appear for the average user.
SoftQuad has introduced Panorama, an SGML browsing tool designed to work in conjunction with Netscape or Mosaic. It works alongside your HTML browser. When you encounter an SGML Web page, Panorama enables you to browse it as easily as an HTML page.
• See “Panorama Pro,” p. 453
Panorama is the first SGML Web browser that works alongside an HTML browser. A free version of it is available on the Web and from FTP sites, and is also included on the CD-ROM included with this book. A full-featured version, called Panorama Pro, is available commercially from SoftQuad. These browsers are discussed in Chapter 25. The SGML home page in figure 1.6 lists links to other SGML-related Internet resources.
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Fig. 1.6 Panorama, by SoftQuad, activates whenever your browser encounters an SGML Web page.
SGML Resources on the Internet
Resources for SGML abound on the Internet, including Gopher sites, FTP sites, Web sites, and chat channels. You can Telnet to SGML BBSes and talk with SGML gurus by using Internet Phone.
Figure 1.7 shows an Archie search on SGML. It turned up 37 hits on one Gopher server in the United Kingdom. Each hit might lead to even more information on SGML documents.
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Fig. 1.7 A sample Archie search on SGML shows 37 hits from just one server. Numerous SGML archives exist.
One newsgroup where gurus hang out is COMP.TEXT.SGML on UseNet. Figure 1.8 shows the range of topics covered on a normal day in this newsgroup.
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Fig. 1.8 The COMP.TEXT.SGML newsgroup is a popular source for detailed information about SGML.
Tip:
There are enough SGML resources on the Internet to bury you. If you want to stay busy for the next few months, check out one of the SGML home pages and trace the links. A popular one is http://www.sil.org/sgml/sgml.html.
This is just a small sample of what is available on the Internet. You can start at any one of these places and quickly find related sources too numerous to count.
What the Future May Hold for SGML and Its Applications
The World Wide Web is just starting out. The first browsers became available only a few years ago. HTML is in its infancy, and already the Web has exploded with growth. Businesses connect to the Web every day. It is hard to underestimate all the changes that will occur over the coming years.
HTML enables the Internet to present information graphically as well as textually. Sound and video clips are also easily available over the Web. The biggest problem with these types of files is they are difficult to transfer over the Internet because of their immense size and bandwidth requirements. Compression technology will enable them to take up no more space than text or small graphics. These types of files are just the beginning.
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