things, SGML was used in the documentation of the stealth bomber. A typical SGML
project results in the creation of a complex HTML-like language. In fact, HTML is an
application of SGML.
Why didnt the Web creators just use SGML? SGML is much too heavyweight for
popular use over the Internet. Though the Web would be a richer place if everyone
used SGML to create their Web sites, no browser vendors were willing to implement
SGML. SGML would also dramatically increase the network burden. Perhaps most
important, SGML is complex to learn, whereas HTML is very simple. Without a simple
markup language like HTML, the Web probably would have never reached critical
So then, whats wrong with HTML? While SGML is too complex, HTML is a bit too
simple for the demands of contemporary Web applications. HTML is intrinsically tied
to how documents should look to their users. Though HTML documents are highly
structured, they are structured around presentation. Consider the H1 tag. The text con-
tained inside of an H1 tag is usually a headline of some sort, but you dont really know
much more about it than that. About all you know is that the Web page author wants
that text to stand out.
Along the same lines, consider Web sites that tell you when the Web site was last
modified. You can look at a table of contents Web page and it will tell you instantly
when the content was last changed. However, there is no way to look at the HTML
code and algorithmically determine when it was last updated. Even though HTML
documents are highly structured, they arent semantically structured. There is no way
to look at HTML and interpret it as much more than text data.
What would work is the ability to define your own tags somehow; however, HTML
doesnt let you do that. The tag set is created by a standards body. Because all of the
browser vendors are expected to implement the latest features of HTML, any additional
functionality needs to be universally applicable. This is understandable, of course.
HTML represents the presentation layer and will never be applicable to the structure of
your data. What is needed, then, is a way to structure your data separate from HTML.
This is the value of XML. Instead of confining yourself to HTML, you are able to cre-
ate your own language for your application. Oracles canonical schema that youll
learn about in a few pages is an example of this. Oracle developed a way to represent
SQL result sets in XML. Once established, XSQL developers can use it to present and
manipulate the data. You also have the power to define your own markup language
and can do it very quickly. You dont have to write a parserXML parsers are widely
available. Your burden is simply to think about what you need, create an XML schema,
and document it. Once created, other people inside and outside of your organization
can use it for data interchange.
NOTE Defining your own tags is great, but an XML document isnt instantly
useful by itself. While an HTML document is immediately usable by millions of
Web browsers worldwide, an XML document isnt. You can create a
<LastUpdated> tag, but what application will understand it? Thus, XML is
primarily used, in conjunction with XSLT, to create HTML and other
standardized document types, or by a Web services client that knows the XML
schema you are using.
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