<element name="addressBook" xmlns="http://relaxng.org/ns/structure/1.0">
  <zeroOrMore>
    <element name="card">
      <element name="name">
        <text/>
      </element>
      <element name="email">
        <text/>
      </element>
      <optional>
        <element name="prefersHTML">
          <empty/>
        </element>
      </optional>
    </element>
  </zeroOrMore>
</element>