• Originally, details of how a WWW page was displayed was left to the browser.
• HTML has evolved: author has more control over the appearance.
• So the HTML of a WWW page is a mix of data and presentation instructions.

<table border=1>
   <tr>
      <td><b>inkjet cartridges</b></td>
      <td><em>HP Deskjet print cartridge 51645A</em></td>
      <td bgcolor="yellow">19.50</td>
   </tr>
...
</table>

• XML is another markup language.
• An XML document does not usually contain presentation details.
• Instead, it is just used to describe data.
• The tags of the language are chosen by the author.
• For this reason, it is called the eXtensible Markup Language.

<consumables>
   <product>
      <category>inkjet cartridges</category>
      <item>HP Deskjet print cartridge 51645A</item>
      <price>19.50</price>
   </product>
   ...
</consumables>

http://www.dur.ac.uk/barry.cornelius/papers/xml.processing.using.php/code/consumables.xml.