We say extensible because it allows us to extend SGML (which is the mother of all metalanguages) and because it is not limited to a certain number of tags like HTML.


What is XML designed for?

XML is designed to make it easy and straightforward to use SGML on the Web. By achieving that, it will be easy to define document types and easy to transmit and share them across the Web. The goal is to enable SGML to be served and available on the Web in the way that it is now possible with HTML.


How do XML and HTML differ?

In order to understand the difference we must understand first what is SGML.

SGML is the "Standard Generalized Markup Language", used for creating thousands of metalanguages
in many fields of human activity, and HTML is one of these languages.

Because SGML is hard to write, XML was introduced making it easier for Web Designers to write SGML.
In other words, XML is an abbreviated version of SGML. It makes it easier for you to define your own
document types, and easier for programmers to write programs to handle them. It omits the more
complex and less-used parts of SGML making it easier to write applications, easier to understand,
and more suited to delivery and interoperability over the Web.


Why is XML so important?

XML is important for two reasons:

1. It eliminates the dependence on one single and inflexible markup language: HTML.
2. It removes the complexity of SGML

"To make SGML simpler, XML redefines some of SGML's internal values and parameters, and removes a large number of the more complex and sometimes less-used features which made it harder to write processing programs. But, it retains all of SGML's structural abilities that allow you to define your own document type." (World Wide Web Consortium, W3C)


Why should I use XML instead of HTML?

• As we already mentioned, authors can design their own document types using XML, instead of being stuck with one document type: HTML.
• XML can provide more and better facilities for browser presentation and performance.
• Information will be more accessible and reusable.
• XML can be used outside the Web as well.
  
  
  

What does an XML document look like?

The basic structure is very similar to HTML. XML documents can be very simple:

<conversation>
<question>How are you?</question>
<answer>I am fine what about you?</answer>
</conversation>

The above example looks just like an HTML file with the exception that we have defined our own tags.

In the previous question, we mentioned that XML files are reusable. As an example of that, a Search Engine may be asked to search for questions in the above XML file. This job is easy, since the Search Engine will search for sentences enclosed between <question>...</question> tags.


How do I tell the browser how to display each tag?

A new Extensible Style Language (XSL) is being proposed for use specifically with XML. The main job of XSL will be to define how each tag will be displayed inside the browser. This will offer another advantage of using XML. Authors will be able to work independently from designers. In other words, the author will write the XML file using his own descriptive tags, and the designer will write the XSL file defining the way each tag will be displayed.


Can I start using XML right now?

Not yet! XML is still under development, but there are already some pilot browsers, such as Internet Explorer 4.0 (Netscape will support XML in its 4.5 version), that support this language, so you can experiment with them. Few months later, when the specification is complete, more software should start to appear and you
will be able to experiment more with them.


Conclusion

Although it is not yet finished, XML promises to offer a powerful language for next-generation web design.
If you are used to authoring and serving HTML, learning XML will be very easy, but at the moment there is
still a need for tutorials, simple tools, and more examples of XML documents. Well-formed XML documents
may look similar to HTML except for some small but very important points of syntax.
As for the answer to the main question, it is obvious that XML is the "Mother" of HTML or the "Son" of SGML.