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 afcontinue="Fig1.38_Packet_Loss_and_Retransmission.gif|3" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:ESP Header Format.gif" />
      <f ns="6" title="File:ESP Position in IPv4 and IPv6.gif" />
      <f ns="6" title="File:Ethernet fields values.gif" />
      <f ns="6" title="File:Ethernet frame with payload.gif" />
      <f ns="6" title="File:Ethernet payload for PPPoE.gif" />
      <f ns="6" title="File:Fig1.11 ARP Transmission Sequence.gif" />
      <f ns="6" title="File:Fig1.12 ARP Message.gif" />
      <f ns="6" title="File:Fig1.21 Special Forms of IP Addresses.gif" />
      <f ns="6" title="File:Fig1.31 Two Levels of ICMP Encapsulation.gif" />
      <f ns="6" title="File:Fig1.37 Simple Positive Acknowledgment with Retransmission.gif" />
    </allfileusages>
  </query>
</api>