Mixed Content

DTD:

   <!ELEMENT p (#PCDATA | b)*>
   <!ELEMENT b (#PCDATA)>

XML Schema:

   <xsd:complexType name="boldText" mixed="true">
      <xsd:sequence>
         <xsd:element name="b" type="xsd:string" 
                    minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
   </xsd:complexType>
   <xsd:element name="p" type="boldText"/>

XML Instance Document:

   <p>This is <b>bold</b> text</p>