hook_theme_suggestions_HOOK() doesn't work I am making a template in Drupal 8. At this moment, I made a theme in the /themes directory; there, I have my .theme file and a copy of the html.html.twig file from the core/themes/stable directory.

I have enabled Twig debugging mode and in the Chrome inspect tool I read the following. 

&lt;!-- FILE NAME SUGGESTIONS:
   * html--front.html.twig
   * html--.html.twig
   x html.html.twig
--&gt;


I added the following hook to the theme file.

function testTheme_suggestions_html_alter(array &amp;$suggestions, array $variables) {
    $suggestions[] = 'html__testHTML';
}


After clearing the cache, there isn't any change to the file name suggestions.

Also I have activated kint() in html.html.twig file that is located in my theme root, but kint() also shows only these variables.

'theme_hook_suggestions' =&gt; array(2)
string(6) "html__"
string(11) "html__front"


Why doesn't the new suggestions appear?
Try it like this:

function yourthemename_theme_suggestions_html_alter(array &amp;$suggestions, array $variables) {
  $suggestions[] = 'html__testHTML';
}


Replace "yourthemename" with your theme name.