unsetting css is not working I am trying to unsetting the deafult CSS through a foreach loop, but seems its not working in hook_preprocess_html(&amp;$variables)

here is my code, please have a look.

$arrCSS = drupal_add_css();
          foreach ($arrCSS as $key => $value) {
            unset($arrCSS[$key]['data']);
          }
          $all_css = drupal_add_css();
          print_r($all_css);
CSS should be manipulated using hook_css_alter(), not in an HTML preprocess (where it's too late).

function hook_css_alter(&amp;$css) {
  // Remove defaults.css file.
  unset($css[drupal_get_path('module', 'system') . '/defaults.css']);
}