Attribute Groups

DTD:

   <!ELEMENT img EMPTY>
   <!ATTLIST img
             alt CDATA #REQUIRED
	     src CDATA #REQUIRED
	     width CDATA #IMPLIED
	     height CDATA #IMPLIED>

XML Schema:

   <xsd:attributeGroup name="imgAttributes">
      <xsd:attribute name="alt" type="xsd:string" use="required"/>
      <xsd:attribute name="src" type="xsd:NMTOKEN" use="required"/>
      <xsd:attribute name="width" type="xsd:integer"/>
      <xsd:attribute name="height" type="xsd:integer"/>
   </xsd:attributeGroup>

   <xsd:element name="img">
      <xsd:complexType>
         <xsd:attributeGroup ref="imgAttributes"/>
      <xsd:complexType>
   </xsd:element>