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>
    <imageusage iucontinue="6|Albert_Einstein_Head.jpg|20" />
  </query-continue>
  <query>
    <imageusage>
      <iu pageid="4" ns="0" title="各班均分" />
      <iu pageid="7" ns="0" title="各班均分表-详细版" />
      <iu pageid="8" ns="6" title="文件:2.png" />
      <iu pageid="10" ns="0" title="特色班平均分" />
      <iu pageid="16" ns="0" title="命题老师" />
    </imageusage>
  </query>
</api>