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>* list=categorymembers (cm) *
  List all pages in a given category

This module requires read rights
Parameters:
  cmtitle             - Which category to enumerate (required). Must include Category: prefix. Cannot be used together with cmpageid
  cmpageid            - Page ID of the category to enumerate. Cannot be used together with cmtitle
  cmprop              - What pieces of information to include
                         ids           - Adds the page ID
                         title         - Adds the title and namespace ID of the page
                         sortkey       - Adds the sortkey used for sorting in the category (hexadecimal string)
                         sortkeyprefix - Adds the sortkey prefix used for sorting in the category (human-readable part of the sortkey)
                         type          - Adds the type that the page has been categorised as (page, subcat or file)
                         timestamp     - Adds the timestamp of when the page was included
                        Values (separate with '|'): ids, title, sortkey, sortkeyprefix, type, timestamp
                        Default: ids|title
  cmnamespace         - Only include pages in these namespaces
                        Values (separate with '|'): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 102, 103, 108, 109
                        Maximum number of values 50 (500 for bots)
  cmtype              - What type of category members to include. Ignored when cmsort=timestamp is set
                        Values (separate with '|'): page, subcat, file
                        Default: page|subcat|file
  cmcontinue          - For large categories, give the value returned from previous query
  cmlimit             - The maximum number of pages to return.
                        No more than 500 (5000 for bots) allowed
                        Default: 10
  cmsort              - Property to sort by
                        One value: sortkey, timestamp
                        Default: sortkey
  cmdir               - In which direction to sort
                        One value: asc, desc, ascending, descending, newer, older
                        Default: ascending
  cmstart             - Timestamp to start listing from. Can only be used with cmsort=timestamp
  cmend               - Timestamp to end listing at. Can only be used with cmsort=timestamp
  cmstartsortkey      - Sortkey to start listing from. Must be given in binary format. Can only be used with cmsort=sortkey
  cmendsortkey        - Sortkey to end listing at. Must be given in binary format. Can only be used with cmsort=sortkey
  cmstartsortkeyprefix - Sortkey prefix to start listing from. Can only be used with cmsort=sortkey. Overrides cmstartsortkey
  cmendsortkeyprefix  - Sortkey prefix to end listing BEFORE (not at, if this value occurs it will not be included!). Can only be used with cmsort=sortkey. Overrides cmendsortkey
Examples:
  Get first 10 pages in [[Category:Physics]]:
    api.php?action=query&list=categorymembers&cmtitle=Category:Physics
  Get page info about first 10 pages in [[Category:Physics]]:
    api.php?action=query&generator=categorymembers&gcmtitle=Category:Physics&prop=info
Help page:
  https://www.mediawiki.org/wiki/API:Categorymembers
</module>
  </help>
</api>