A.4. Validity

In addition to well-formedness, XML 1.0 offers another level of verification, called validity. To explain why validity is important, let's take a simple example. Imagine you invented a simple XML format for your friends' telephone numbers:

<phonebook>
  <person>
    <name>Albert Smith</name>
    <number>123-456-7890</number>
  </person>
  <person>
    <name>Bertrand Jones</name>
    <number>456-123-9876</number>
  </person>
</phonebook>