hook_preprocess_html(&$vars) not firing I am attempting to add css classes to the body depoendant on what the page contains.

I am using the following...

function myTheme_preprocess_html(&amp;$vars){
    //code here
    //eg.
    $vars['classes_array'][] = 'a-new-class-for-a-node';
}


I cannot get this to fire or add anything to the page. It will not drupal_set_message, echo or more importantly add any classes to the body tag.

What am I doing wrong here please? I am using Drupal 6.
I might be terribly wrong here, but there appears to be no such hook for Drupal 6. If so, it will never fire. 

See https://api.drupal.org/api/drupal/6/search/hook_preprocess_html.

Perhaps you're backporting a theme from D7?