Issue #2368739 by Wim Leers: Fixed user_page_attachments() unnecessarily adds an individual CSS file on every page.

core/modules/user/css/user.admin.css

@@ -0,0 +1,24 @@
+/**
+ * @file
+ * Admin styling for the User module.
+ */
+
+#permissions td.module {
+  font-weight: bold;
+}
+#permissions td.permission {
+  padding-left: 1.5em; /* LTR */
+}
+[dir="rtl"] #permissions td.permission {
+  padding-left: 0;
+  padding-right: 1.5em;
+}
+#permissions tr.odd .form-item,
+#permissions tr.even .form-item {
+  white-space: normal;
+}
+
+#user-admin-settings .details-description {
+  font-size: 0.85em;
+  padding-bottom: .5em;
+}

core/modules/user/css/user.module.css

@@ -1,22 +1,3 @@
-#permissions td.module {
-  font-weight: bold;
-}
-#permissions td.permission {
-  padding-left: 1.5em; /* LTR */
-}
-[dir="rtl"] #permissions td.permission {
-  padding-left: 0;
-  padding-right: 1.5em;
-}
-#permissions tr.odd .form-item,
-#permissions tr.even .form-item {
-  white-space: normal;
-}
-#user-admin-settings .details-description {
-  font-size: 0.85em;
-  padding-bottom: .5em;
-}
-
 /**
  * Password strength indicator.
  */

core/modules/user/src/AccountSettingsForm.php

@@ -65,6 +65,8 @@ class AccountSettingsForm extends ConfigFormBase {
     $mail_config = $this->config('user.mail');
     $site_config = $this->config('system.site');
 
+    $form['#attached']['library'][] = 'user/drupal.user.admin';
+
     // Settings for anonymous users.
     $form['anonymous_settings'] = array(
       '#type' => 'details',

core/modules/user/user.libraries.yml

@@ -10,6 +10,12 @@ drupal.user:
     - core/drupal
     - core/jquery.once
 
+drupal.user.admin:
+  version: VERSION
+  css:
+    theme:
+      css/user.admin.css: {}
+
 drupal.user.permissions:
   version: VERSION
   js:
@@ -19,6 +25,7 @@ drupal.user.permissions:
     - core/jquery.once
     - core/drupal
     - core/drupalSettings
+    - user/drupal.user.admin
 
 drupal.user.icons:
   version: VERSION

core/modules/user/user.module

@@ -107,14 +107,6 @@ function user_theme() {
   );
 }
 
-/**
- * Implements hook_page_attachments().
- */
-function user_page_attachments(array &$page) {
-  $path = drupal_get_path('module', 'user');
-  $page['#attached']['css'][$path . '/css/user.module.css'] = array('every_page' => TRUE);
-}
-
 /**
  * Implements hook_js_alter().
  */