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To add CSS to node edit pages you can use hook_form_alter. Then check the $form_id to localize changes to desired node form(s) and add you call to drupal_add_css().

Example:

function admincss_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == "xxx") {
    drupal_add_css(drupal_get_path('module', 'admincss') . '/css/admincss.css');
  }
}

hook_preprocess_html() Is used to preprocess variables for the html.tpl.php file.