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>
    <allfileusages gafcontinue="Barringtonensis-map.jpg|2189" />
  </query-continue>
  <query>
    <pages>
      <page pageid="537" ns="0" title="Acacia baileyana" />
      <page pageid="814" ns="0" title="Fumarates to Freebase Conversion TEK" />
      <page pageid="2181" ns="0" title="Acacia baeuerlenii" />
      <page pageid="2182" ns="0" title="Acacia bakeri" />
      <page pageid="2183" ns="0" title="Acacia balsamea" />
      <page pageid="2184" ns="0" title="Acacia bancroftiorum" />
      <page pageid="2185" ns="0" title="Acacia barakulensis" />
      <page pageid="2186" ns="0" title="Acacia barattensis" />
      <page pageid="2187" ns="0" title="Acacia barbinervis" />
      <page pageid="2188" ns="0" title="Acacia barrettiorum" />
    </pages>
  </query>
</api>