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An example: WidgetML with namespaces

<widget xmlns=""
  <head size="medium"/>
  <big><subwidget ref="gizmo"/></big>
      <xhtml:title>Description of gadget</xhtml:title>
      A gadget contains a big gizmo

Now the main part of WidgetML uses the default namespace which has the URI;
XHTML uses the namespace prefix xhtml which is assigned the URI