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>
  <help>
    <module>* meta=siteinfo (si) *
  Return general information about the site

This module requires read rights
Parameters:
  siprop              - Which sysinfo properties to get:
                         general               - Overall system information
                         namespaces            - List of registered namespaces and their canonical names
                         namespacealiases      - List of registered namespace aliases
                         specialpagealiases    - List of special page aliases
                         magicwords            - List of magic words and their aliases
                         statistics            - Returns site statistics
                         interwikimap          - Returns interwiki map (optionally filtered, (optionally localised by using siinlanguagecode))
                         dbrepllag             - Returns database server with the highest replication lag
                         usergroups            - Returns user groups and the associated permissions
                         extensions            - Returns extensions installed on the wiki
                         fileextensions        - Returns list of file extensions allowed to be uploaded
                         rightsinfo            - Returns wiki rights (license) information if available
                         languages             - Returns a list of languages MediaWiki supports (optionally localised by using siinlanguagecode)
                         skins                 - Returns a list of all enabled skins
                         extensiontags         - Returns a list of parser extension tags
                         functionhooks         - Returns a list of parser function hooks
                         showhooks             - Returns a list of all subscribed hooks (contents of $wgHooks)
                         variables             - Returns a list of variable IDs
                        Values (separate with '|'): general, namespaces, namespacealiases, specialpagealiases, magicwords, interwikimap, dbrepllag,
                            statistics, usergroups, extensions, fileextensions, rightsinfo, languages, skins, extensiontags,
                            functionhooks, showhooks, variables
                        Default: general
  sifilteriw          - Return only local or only nonlocal entries of the interwiki map
                        One value: local, !local
  sishowalldb         - List all database servers, not just the one lagging the most
  sinumberingroup     - Lists the number of users in user groups
  siinlanguagecode    - Language code for localised language names (best effort, use CLDR extension)
Examples:
  api.php?action=query&meta=siteinfo&siprop=general|namespaces|namespacealiases|statistics
  api.php?action=query&meta=siteinfo&siprop=interwikimap&sifilteriw=local
  api.php?action=query&meta=siteinfo&siprop=dbrepllag&sishowalldb=
Help page:
  https://www.mediawiki.org/wiki/API:Meta#siteinfo_.2F_si
</module>
  </help>
</api>