HTML is the most successful markup language of all time. You can view the simplest HTML tags on virtually any device, from palmtops to mainframes, and you can even convert HTML markup into voice and other formats with the right tools. Given the success of HTML, why did the W3C create XML? To answer that question, take a look at this document:

<p><b>Mrs. Mary McGoon</b>
<br>
1401 Main Street
<br>
Anytown, NC 34829</p>
          

The trouble with HTML is that it was designed with humans in mind. Even without viewing the above HTML document in a browser, you and I can figure out that it is someone's postal address. (Specifically, it's a postal address for someone in the United States; even if you're not familiar with all the components of U.S. postal addresses, you could probably guess what this represents.)

As humans, you and I have the intelligence to understand the meaning and intent of most documents. A machine, unfortunately, can't do that. While the tags in this document tell a browser how to display this information, the tags don't tell the browser what the information is. You and I know it's an address, but a machine doesn't.