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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>
    <allmessages>
      <message name="ipb-blockingself" xml:space="preserve">You are about to block yourself! Are you sure you want to do that?</message>
      <message name="ipb-blocklist" xml:space="preserve">View existing blocks</message>
      <message name="ipb-blocklist-contribs" xml:space="preserve">Contributions for $1</message>
      <message name="ipb-change-block" xml:space="preserve">Re-block the user with these settings</message>
      <message name="ipb-confirm" xml:space="preserve">Confirm block</message>
      <message name="ipb-confirmaction" xml:space="preserve">If you are sure you really want to do it, please check the &quot;{{int:ipb-confirm}}&quot; field at the bottom.</message>
      <message name="ipb-confirmhideuser" xml:space="preserve">You are about to block a user with &quot;hide user&quot; enabled. This will suppress the user&#039;s name in all lists and log entries. Are you sure you want to do that?</message>
      <message name="ipb-default-expiry" xml:space="preserve" />
      <message name="ipb-disableusertalk" xml:space="preserve">Prevent this user from editing their own talk page while blocked</message>
      <message name="ipb-edit-dropdown" xml:space="preserve">Edit block reasons</message>
      <message name="ipb-hardblock" xml:space="preserve">Prevent logged-in users from editing from this IP address</message>
      <message name="ipb-needreblock" xml:space="preserve">$1 is already blocked. Do you want to change the settings?</message>
      <message name="ipb-otherblocks-header" xml:space="preserve">Other {{PLURAL:$1|block|blocks}}</message>
      <message name="ipb-unblock" xml:space="preserve">Unblock a username or IP address</message>
      <message name="ipb-unblock-addr" xml:space="preserve">Unblock $1</message>
    </allmessages>
  </query>
</api>