Issue #1938430 by shanethehat, steveoliver, Fabianx: Don't add a default theme hook class in template_preprocess().

core/includes/theme.inc

@@ -1646,6 +1646,10 @@ function template_preprocess_datetime(&$variables) {
       $variables['html'] = FALSE;
     }
   }
+
+  // Add a 'datetime' class.
+  $variables['attributes']['class'][] = 'datetime';
+
   $variables['attributes'] = new Attribute($variables['attributes']);
 }
 
@@ -2686,9 +2690,6 @@ function template_preprocess(&$variables, $hook) {
     'title_attributes' => array(),
     'content_attributes' => array(),
   );
-
-  // Initialize html class attribute for the current hook.
-  $variables['attributes']['class'][] = drupal_html_class($hook);
 }
 
 /**
@@ -2732,6 +2733,7 @@ function template_preprocess_html(&$variables) {
 
   // Compile a list of classes that are going to be applied to the body element.
   // This allows advanced theming based on context (home page, node of certain type, etc.).
+  $variables['attributes']['class'][] = 'html';
   // Add a class that tells us whether we're on the front page or not.
   $variables['attributes']['class'][] = $variables['is_front'] ? 'front' : 'not-front';
   // Add a class that tells us whether the page is viewed by an authenticated user or not.
@@ -3109,6 +3111,7 @@ function template_preprocess_maintenance_page(&$variables) {
   $variables['title']             = drupal_get_title();
 
   // Compile a list of classes that are going to be applied to the body element.
+  $variables['attributes']['class'][] = 'maintenance-page';
   $variables['attributes']['class'][] = 'in-maintenance';
   if (isset($variables['db_is_active']) && !$variables['db_is_active']) {
     $variables['attributes']['class'][] = 'db-offline';
@@ -3169,6 +3172,7 @@ function template_preprocess_region(&$variables) {
   $variables['content'] = $variables['elements']['#children'];
   $variables['region'] = $variables['elements']['#region'];
 
+  $variables['attributes']['class'][] = 'region';
   $variables['attributes']['class'][] = drupal_region_class($variables['region']);
   $variables['theme_hook_suggestions'][] = 'region__' . $variables['region'];
 }

core/modules/block/block.module

@@ -539,6 +539,7 @@ function template_preprocess_block(&$variables) {
   $variables['label'] = !empty($variables['configuration']['label_display']) ? $variables['configuration']['label'] : '';
   $variables['content'] = $variables['elements']['content'];
 
+  $variables['attributes']['class'][] = 'block';
   $variables['attributes']['class'][] = drupal_html_class('block-' . $variables['configuration']['module']);
 
   // Add default class for block content.

core/modules/comment/comment.module

@@ -1646,6 +1646,7 @@ function template_preprocess_comment(&$variables) {
   }
 
   // Gather comment classes.
+  $variables['attributes']['class'][] = 'comment';
   // 'published' class is not needed, it is either 'preview' or 'unpublished'.
   if ($variables['status'] != 'published') {
     $variables['attributes']['class'][] = $variables['status'];
@@ -1724,6 +1725,9 @@ function template_preprocess_comment_wrapper(&$variables) {
   $variables['display_mode'] = variable_get('comment_default_mode_' . $variables['node']->type, COMMENT_MODE_THREADED);
   // The comment form is optional and may not exist.
   $variables['content'] += array('comment_form' => array());
+
+  // Add a comment wrapper class.
+  $variables['attributes']['class'][] = 'comment-wrapper';
 }
 
 /**

core/modules/layout/lib/Drupal/layout/Plugin/Layout/StaticLayout.php

@@ -82,6 +82,9 @@ class StaticLayout extends PluginBase implements LayoutInterface {
     $build = array(
       '#theme' => $definition['theme'],
       '#content' => array(),
+      '#attributes' => array(
+        'class' => drupal_html_class($definition['theme']),
+      ),
     );
 
     // Render all regions needed for this layout.

core/modules/node/node.module

@@ -1186,6 +1186,7 @@ function template_preprocess_node(&$variables) {
   $variables['attributes']['role'] = 'article';
 
   // Gather node classes.
+  $variables['attributes']['class'][] = 'node';
   $variables['attributes']['class'][] = drupal_html_class('node-' . $node->type);
   if ($node->promote) {
     $variables['attributes']['class'][] = 'promoted';

core/modules/overlay/overlay.module

@@ -519,6 +519,10 @@ function template_preprocess_overlay(&$variables) {
   $variables['tabs'] = menu_primary_local_tasks();
   $variables['title'] = drupal_get_title();
   $variables['disable_overlay'] = overlay_disable_message();
+
+  // Add atrributes for the overlay container.
+  $variables['attributes']['class'][] = 'overlay';
+  // Add attributes for the overlay content.
   $variables['content_attributes']['class'][] = 'clearfix';
 }
 

core/modules/taxonomy/taxonomy.module

@@ -490,6 +490,7 @@ function template_preprocess_taxonomy_term(&$variables) {
   field_attach_preprocess($term, $variables['content'], $variables);
 
   // Gather classes, and clean up name so there are no underscores.
+  $variables['attributes']['class'][] = 'taxonomy-term';
   $vocabulary_name_css = str_replace('_', '-', $term->bundle());
   $variables['attributes']['class'][] = 'vocabulary-' . $vocabulary_name_css;
 

core/modules/views/views.theme.inc

@@ -620,6 +620,7 @@ function template_preprocess_views_view_table(&$vars) {
   }
 
   $vars['attributes']['class'][] = 'views-table';
+  $vars['attributes']['class'][] = 'views-view-table';
   if (empty($vars['rows']) && !empty($options['empty_table'])) {
     $build = $view->display_handler->renderArea('empty');
     $vars['rows'][0][0] = drupal_render($build);

core/modules/views_ui/views_ui.theme.inc

@@ -24,6 +24,8 @@ function template_preprocess_views_ui_display_tab_setting(&$variables) {
   array_unshift($variables['settings_links'], $variables['link']);
   $variables['settings_links'] = implode('<span class="label">&nbsp;|&nbsp;</span>', $variables['settings_links']);
 
+  $variables['attributes']['class'][] = 'views-ui-display-tab-setting';
+
   if (!empty($variables['defaulted'])) {
     $variables['attributes']['class'][] = 'defaulted';
   }
@@ -41,6 +43,8 @@ function template_preprocess_views_ui_display_tab_setting(&$variables) {
 function template_preprocess_views_ui_display_tab_bucket(&$variables) {
   $element = $variables['element'];
 
+  $variables['attributes']['class'][] = 'views-ui-display-tab-bucket';
+
   if (!empty($element['#name'])) {
     $variables['attributes']['class'][] = drupal_html_class($element['#name']);
   }