How to override the default zen html template depending on a node type?
Jonathan, your missing an underscore...

function test_preprocess_html(&amp;$vars){
  if (!empty($vars['node'])) {
    $vars['theme_hook_suggestions'][] = 'html__node__' . $vars['node']-&gt;type;
  }
}


Your get it if you see whats suggested already be doing a var_dump($vars).

This has nothing to do with the Zen theme, it's just core functionality.

To understand how this is formed you can take a look at the theme_get_suggestions() function in theme.inc

/**
 * Generate an array of suggestions from path arguments.
 *
 * This is typically called for adding to the 'theme_hook_suggestions' or
 * 'classes_array' variables from within preprocess functions, when wanting to
 * base the additional suggestions on the path of the current page.
 *
 * @param $args
 *   An array of path arguments, such as from function arg().
 * @param $base
 *   A string identifying the base 'thing' from which more specific suggestions
 *   are derived. For example, 'page' or 'html'.
 * @param $delimiter
 *   The string used to delimit increasingly specific information. The default
 *   of '__' is appropriate for theme hook suggestions. '-' is appropriate for
 *   extra classes.
 *
 * @return
 *   An array of suggestions, suitable for adding to
 *   $variables['theme_hook_suggestions'] within a preprocess function or to
 *   $variables['classes_array'] if the suggestions represent extra CSS classes.
 */
function theme_get_suggestions($args, $base, $delimiter = '__') {

  // Build a list of suggested theme hooks or body classes in order of
  // specificity. One suggestion is made for every element of the current path,
  // though numeric elements are not carried to subsequent suggestions. For
  // example, for $base='page', http://www.example.com/node/1/edit would result
  // in the following suggestions and body classes:
  //
  // page__node              page-node
  // page__node__%           page-node-%
  // page__node__1           page-node-1
  // page__node__edit        page-node-edit

  $suggestions = array();
  $prefix = $base;
  foreach ($args as $arg) {
    // Remove slashes or null per SA-CORE-2009-003 and change - (hyphen) to _
    // (underscore).
    //
    // When we discover templates in @see drupal_find_theme_templates,
    // hyphens (-) are converted to underscores (_) before the theme hook
    // is registered. We do this because the hyphens used for delimiters
    // in hook suggestions cannot be used in the function names of the
    // associated preprocess functions. Any page templates designed to be used
    // on paths that contain a hyphen are also registered with these hyphens
    // converted to underscores so here we must convert any hyphens in path
    // arguments to underscores here before fetching theme hook suggestions
    // to ensure the templates are appropriately recognized.
    $arg = str_replace(array("/", "\\", "\0", '-'), array('', '', '', '_'), $arg);
    // The percent acts as a wildcard for numeric arguments since
    // asterisks are not valid filename characters on many filesystems.
    if (is_numeric($arg)) {
      $suggestions[] = $prefix . $delimiter . '%';
    }
    $suggestions[] = $prefix . $delimiter . $arg;
    if (!is_numeric($arg)) {
      $prefix .= $delimiter . $arg;
    }
  }
  if (drupal_is_front_page()) {
    // Front templates should be based on root only, not prefixed arguments.
    $suggestions[] = $base . $delimiter . 'front';
  }

  return $suggestions;
}


Have a look at the $delimter argument. The default is '__' for theme_hook_suggestions().