AlternateContent Syntax

<AlternateContent>
  <Choice Requires="namespacefoo">
    <Somemarkup/>
  <Choice Requires="namespacefoo namespacefoobar">
    <Somealternatemarkup/>
  <Fallback>
    <Choiceoflastresort/>
</AlternateContent>

The <AlternateContent> element and its children, <Choice> and <Fallback>, are used to provide alternates for specified content. Each <Choice> element is examined in turn. The @Requires attribute specifies a set of space-delimited namespaces that must be understood in order to select that choice. If there is a match between required namespaces and what the consumer understands, the appropriate <Choice> is returned. If there are multiple possible matches, only the first match is returned. An optional <Fallback> element can be used, and is utilized as a default when no match occurs.