This page has connections to several pages that deal with XML, HTML and XHTML and attempts to define the differences between the three technologies.
This introduction starts with an explanation of XML, done by the experts at W3C in there page - XML in 10 Points.
Once you have reviewed the experts introduction, you may want to see an overview of some of the XML technologies that are present today. Since this overview looks at the broad category of XML, and Netscape 4.7 does not support the technology, you should use Internet Explorer when looking at the overview.
Once you are familiar with the rules for XML, the next step is to dentify the specific rules for XHTML and how they extend the basic rules to incorporate XHTML specific needs. The entire set of rules is available as a PDF file.
I have also posted a copy of the student instructions as a PDF form. Feel free to copy this form and distribute it to your students.
The last step to this introduction will be to create an XHTML compliant document as a group.