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-continue>
    <exturlusage euoffset="10" />
  </query-continue>
  <query>
    <exturlusage>
      <eu pageid="3670" ns="4" title="Project:Editing Discussion/Archive 01" url="http://www.mediawiki.org" />
      <eu pageid="7541" ns="5" title="Project talk:Languages/ja" url="http://www.MediaWiki.org" />
      <eu pageid="15847" ns="3" title="User talk:Oracle Weatherwax" url="http://www.mediawiki.org/" />
      <eu pageid="24222" ns="9" title="MediaWiki talk:Breadcrumbs" url="http://www.mediawiki.org/wiki/Extension:BreadCrumbs2" />
      <eu pageid="126552" ns="1" title="Talk:LSL Functions by Subject" url="http://www.mediawiki.org/wiki/Extension:CategoryTree" />
      <eu pageid="48023" ns="3" title="User talk:Which Linden" url="http://www.mediawiki.org/wiki/Extension:Cite/Cite.php" />
      <eu pageid="63403" ns="10" title="Template:Footnote" url="http://www.mediawiki.org/wiki/Extension:Cite/Cite.php" />
      <eu pageid="66092" ns="12" title="Help:Extensions" url="http://www.mediawiki.org/wiki/Extension:Cite/Cite.php" />
      <eu pageid="37823" ns="12" title="Help:Editors Toolbox" url="http://www.mediawiki.org/wiki/Extension:Cite/Special:Cite.php" />
      <eu pageid="13941" ns="10" title="Template:Footnote/old" url="http://www.mediawiki.org/wiki/Extension:Cite/Special:Cite.php" />
    </exturlusage>
  </query>
</api>