You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query>
    <exturlusage>
      <eu pageid="261" ns="10" title="Template:Draw graphviz concept relation/doc" url="http://www.mediawiki.org/wiki/Extension:GraphViz" />
      <eu pageid="260" ns="10" title="Template:Draw graphviz concept relation" url="http://www.mediawiki.org/wiki/Extension:GraphViz" />
      <eu pageid="235" ns="10" title="Template:Concept scheme class/doc" url="http://www.mediawiki.org/wiki/Extension:Semantic_Drilldown" />
      <eu pageid="234" ns="10" title="Template:Concept scheme class" url="http://www.mediawiki.org/wiki/Extension:Semantic_Drilldown" />
      <eu pageid="209" ns="10" title="Template:Generate concept formlinks from SMW import page/doc" url="http://www.mediawiki.org/wiki/Extension:StringFunctions" />
      <eu pageid="208" ns="10" title="Template:Generate concept formlinks from SMW import page" url="http://www.mediawiki.org/wiki/Extension:StringFunctions" />
      <eu pageid="209" ns="10" title="Template:Generate concept formlinks from SMW import page/doc" url="http://www.mediawiki.org/wiki/Extension:Variables" />
      <eu pageid="217" ns="10" title="Template:Concept relation/doc" url="http://www.mediawiki.org/wiki/Extension:Variables" />
      <eu pageid="208" ns="10" title="Template:Generate concept formlinks from SMW import page" url="http://www.mediawiki.org/wiki/Extension:Variables" />
      <eu pageid="216" ns="10" title="Template:Concept relation" url="http://www.mediawiki.org/wiki/Extension:Variables" />
    </exturlusage>
  </query>
  <query-continue>
    <exturlusage euoffset="10" />
  </query-continue>
</api>