How do I define a custom html.html.twig file?
If you want to return 'render element' this would be 'html':

'render element' =&gt; 'html',


But you probably don't need this, better return the base hook so that drupal fetches the variables from the hook html:

function mymodule_theme() {
  return array(
    'html__ajax' =&gt; array(
      'base hook' =&gt; 'html',
    ),
  );
}