diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 0549702..e8220e2 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -273,9 +273,6 @@ function template_preprocess_block(&$variables) {
     $variables['label'] = $variables['elements']['content']['#title'];
   }
 
-  $variables['attributes']['class'][] = 'block';
-  $variables['attributes']['class'][] = drupal_html_class('block-' . $variables['configuration']['provider']);
-
   // Add default class for block content.
   $variables['content_attributes']['class'][] = 'content';
 
diff --git a/core/modules/block/src/Tests/BlockViewBuilderTest.php b/core/modules/block/src/Tests/BlockViewBuilderTest.php
index 58becb0..8310550 100644
--- a/core/modules/block/src/Tests/BlockViewBuilderTest.php
+++ b/core/modules/block/src/Tests/BlockViewBuilderTest.php
@@ -82,7 +82,7 @@ public function testBasicRendering() {
     $entity = entity_load('block', 'test_block1');
     $output = entity_view($entity, 'block');
     $expected = array();
-    $expected[] = '<div class="block block-block-test" id="block-test-block1">';
+    $expected[] = '<div id="block-test-block1" class="block block-block-test">';
     $expected[] = '  ';
     $expected[] = '    ';
     $expected[] = '';
@@ -109,7 +109,7 @@ public function testBasicRendering() {
     $entity->save();
     $output = entity_view($entity, 'block');
     $expected = array();
-    $expected[] = '<div class="block block-block-test" id="block-test-block2">';
+    $expected[] = '<div id="block-test-block2" class="block block-block-test">';
     $expected[] = '  ';
     $expected[] = '      <h2>Powered by Bananas</h2>';
     $expected[] = '    ';
diff --git a/core/modules/block/templates/block.html.twig b/core/modules/block/templates/block.html.twig
index 3c51d46..b628cb0 100644
--- a/core/modules/block/templates/block.html.twig
+++ b/core/modules/block/templates/block.html.twig
@@ -21,13 +21,6 @@
  * - content: The content of this block.
  * - attributes: HTML attributes for the containing element.
  *   - id: A valid HTML ID and guaranteed unique.
- *   - class: Classes that can be used to style contextually through
- *     CSS. These can be manipulated through preprocess functions. The default
- *     values can be one or more of the following:
- *     - block: The current template type, i.e., "theming hook".
- *     - block-[module]: The module generating the block. For example, the user
- *       module is responsible for handling the default user navigation block.
- *       In that case the class would be 'block-user'.
  * - title_attributes: HTML attributes for the title element.
  * - content_attributes: HTML attributes for the content element.
  * - title_prefix: Additional output populated by modules, intended to be
@@ -41,7 +34,13 @@
  */
  @todo Remove the div around content as per http://drupal.org/node/1972122.
 #}
-<div{{ attributes }}>
+{%
+  set classes = [
+    'block',
+    'block-' ~ configuration.provider|clean_class,
+  ]
+%}
+<div{{ attributes.addClass(classes) }}>
   {{ title_prefix }}
   {% if label %}
     <h2{{ title_attributes }}>{{ label }}</h2>