(XML) means a post about XML (see Gregor Hagedorn's post, "First level topics on this list (GEN)"; every post should bear a tag among RQT, XML, GEN).

By the way, this kind of tagging is much in the spirit of XML. Even in an HTML text, you can mark up a sentence, a paragraph, or words, with tags like <requirement>, <opinion>, <example>, <reference-to-article>, etc. To identify this family of tags (called a vocabulary), it is better (though not mandatory) to indicate that this fragment of HTML text is using this vocabulary (say, in this example, called authoring-discussion).

Here is a complete example:

<HTML xmlns:disc="urn:authoring-discussion" >
<STYLE>
disc\:opinion{color:red; font-size:large;}
</STYLE>
<P>
Everybody agrees that ......; but I think that <disc:opinion>modelizing amphibians is very
important and a very special case</disc:opinion>.
</P>
</HTML>

You can copy-and-paste this into an example.html file, and see it in Internet Explorer 5, or Mozilla, or any XHTML+CSS1 compliant browser.
For your conveniance, this file is joined to the post.

Notes:
N1 xmlns is a reserved word meaning "XML namespace"; the string following = can be:

a URN, that is an Unique Resource Name, (see RFC 2396, it's only required to remain globally unique and persistent ) as in this example,
or an URL (Uniform Resource Locator), that is something retrievable on the Internet, like http://www.w3.org/DTD/authoring-discussion.dtd , which in our example would contain a formal description of the vocabulary, a DTD (XML Data Type Definition) or another syntax like XML Schema.

N2 This is a CSS1 style; complete specification in http://www.w3.org/TR/REC-CSS1

There many topics more about XML, like:

how and why mix several vocabularies,
how to format and query an XML document (XSLT),
what's going on in the business and scientific domains around XML
relations beetwen XML and HTML,
XML and the Web: the so-called semantic Web.

If time and popular demand are here, I could write more on this.

Cheers

Jean-Marc