diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 4c0e580..6996a8c 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -3226,12 +3226,16 @@ function drupal_common_theme() {
     ),
     'task_list' => array(
       'variables' => array('items' => NULL, 'active' => NULL),
+      'preprocess functions' => array('template_preprocess_task_list'),
+      'template' => 'task-list',
     ),
     'authorize_message' => array(
       'variables' => array('message' => NULL, 'success' => TRUE),
+      'template' => 'authorize-message',
     ),
     'authorize_report' => array(
       'variables' => array('messages' => array()),
+      'template' => 'authorize-report',
     ),
     // From pager.inc.
     'pager' => array(
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index cda4109..6d49b56 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -5,6 +5,8 @@
  * Theming for maintenance pages.
  */
 
+use Drupal\Core\Template\Attribute;
+
 /**
  * Sets up the theming system for maintenance page.
  *
@@ -107,42 +109,38 @@ function _theme_load_offline_registry($theme, $base_theme = NULL, $theme_engine
 }
 
 /**
- * Returns HTML for a list of maintenance tasks to perform.
+ * Prepares variables for maintenance task list templates.
  *
- * @param $variables
+ * Default template: task-list.html.twig.
+ *
+ * @param array $variables
  *   An associative array containing:
  *   - items: An associative array of maintenance tasks.
  *   - active: The key for the currently active maintenance task.
- *
- * @ingroup themeable
  */
-function theme_task_list($variables) {
+function template_preprocess_task_list(&$variables) {
   $t = get_t();
   $items = $variables['items'];
   $active = $variables['active'];
 
   $done = isset($items[$active]) || $active == NULL;
-  $output = '<h2 class="element-invisible">Installation tasks</h2>';
-  $output .= '<ol class="task-list">';
+  $tasks = array();
 
   foreach ($items as $k => $item) {
+    $tasks[$k]['item'] = $item;
+    $tasks[$k]['attributes'] = new Attribute(array('class' => array()));
     if ($active == $k) {
-      $class = 'active';
-      $status = '(' . $t('active') . ')';
+      $tasks[$k]['attributes']['class'][] = 'active';
+      $tasks[$k]['status'] = $t('active');
       $done = FALSE;
     }
     else {
-      $class = $done ? 'done' : '';
-      $status = $done ? '(' . $t('done') . ')' : '';
+      $tasks[$k]['attributes']['class'][] = $done ? 'done' : '';
+      $tasks[$k]['status'] = $done ? $t('done') : '';
     }
-    $output .= '<li';
-    $output .= ($class ? ' class="' . $class . '"' : '') . '>';
-    $output .= $item;
-    $output .= ($status ? '<span class="element-invisible">' . $status . '</span>' : '');
-    $output .= '</li>';
   }
-  $output .= '</ol>';
-  return $output;
+
+  $variables['tasks'] = $tasks;
 }
 
 /**
@@ -176,52 +174,43 @@ function theme_update_page($variables) {
 }
 
 /**
- * Returns HTML for a results report of an operation run by authorize.php.
+ * Prepares variables for authorize.php operation report templates.
  *
- * @param $variables
+ * This report displays the results of an operation run via authorize.php.
+ *
+ * Default template: authorize-report.html.twig.
+ *
+ * @param array $variables
  *   An associative array containing:
  *   - messages: An array of result messages.
- *
- * @ingroup themeable
  */
-function theme_authorize_report($variables) {
+function template_preprocess_authorize_report(&$variables) {
   $messages = $variables['messages'];
-  $output = '';
+  // Top level attributes.
+  $attributes = $variables['attributes'] ? $variables['attributes'] : array();
+  $variables['attributes'] = new Attribute($attributes);
+
   if (!empty($messages)) {
-    $output .= '<div id="authorize-results">';
+    $lists = array();
+    $variables['attributes']['id'] = 'authorize-results';
     foreach ($messages as $heading => $logs) {
       $items = array();
       foreach ($logs as $number => $log_message) {
         if ($number === '#abort') {
           continue;
         }
-        $items[] = theme('authorize_message', array('message' => $log_message['message'], 'success' => $log_message['success']));
+        $items[] = array(
+          '#theme' => 'authorize_message',
+          '#message' => $log_message['message'],
+          '#success' => $log_message['success'],
+        );
       }
-      $output .= theme('item_list',  array('items' => $items, 'title' => $heading));
+      $lists[] = array(
+        '#theme' => 'item_list',
+        '#items' => $items,
+        '#title' => $heading,
+      );
     }
-    $output .= '</div>';
-  }
-  return $output;
-}
-
-/**
- * Returns HTML for a single log message from the authorize.php batch operation.
- *
- * @param $variables
- *   An associative array containing:
- *   - message: The log message.
- *   - success: A boolean indicating failure or success.
- *
- * @ingroup themeable
- */
-function theme_authorize_message($variables) {
-  $message = $variables['message'];
-  $success = $variables['success'];
-  if ($success) {
-    $item = array('data' => $message, 'class' => array('success'));
-  }
-  else {
-    $item = array('data' => '<strong>' . $message . '</strong>', 'class' => array('failure'));
+    $variables['report'] = $lists;
   }
-  return $item;
 }
diff --git a/core/modules/system/templates/authorize-message.html.twig b/core/modules/system/templates/authorize-message.html.twig
new file mode 100644
index 0000000..55623dc
--- /dev/null
+++ b/core/modules/system/templates/authorize-message.html.twig
@@ -0,0 +1,22 @@
+{#
+/**
+ * @file
+ * Default theme implementation for a log message in an authorize.php report.
+ *
+ * This template displays a single message in an authorize.php batch operation
+ * report.
+ *
+ * Available variables:
+ * - message: The log message.
+ * - success: A flag indicating failure or success.
+ *
+ * @see template_preprocess()
+ *
+ * @ingroup themeable
+ */
+#}
+{% if success %}
+  <span class="success">{{ message }}</span>
+{% else %}
+  <span class="failure"><strong>{{ message }}</strong></span>
+{% endif %}
diff --git a/core/modules/system/templates/authorize-report.html.twig b/core/modules/system/templates/authorize-report.html.twig
new file mode 100644
index 0000000..09bcdf0
--- /dev/null
+++ b/core/modules/system/templates/authorize-report.html.twig
@@ -0,0 +1,24 @@
+{#
+/**
+ * @file
+ * Default theme implementation for authorize.php operation report templates.
+ *
+ * This report displays the results of an operation run via authorize.php.
+ *
+ * Available variables:
+ * - report: A list of result messages.
+ * - attributes: HTML attributes for the element.
+ *
+ * @see template_preprocess()
+ * @see template_preprocess_authorize_report()
+ *
+ * @ingroup themeable
+ */
+#}
+{% if report %}
+  <div{{ attributes }}>
+    {% for messages in report %}
+      {{ messages }}
+    {% endfor %}
+  </div>
+{% endif %}
diff --git a/core/modules/system/templates/maintenance-page.html.twig b/core/modules/system/templates/maintenance-page.html.twig
new file mode 100644
index 0000000..a57b546
--- /dev/null
+++ b/core/modules/system/templates/maintenance-page.html.twig
@@ -0,0 +1,92 @@
+{#
+/**
+ * @file
+ * Default theme implementation to display a single Drupal page while offline.
+ *
+ * All the available variables are mirrored in html.html.twig and
+ * page.html.twig.
+ * Some may be blank but they are provided for consistency.
+ *
+ * @see template_preprocess()
+ * @see template_preprocess_maintenance_page()
+ *
+ * @ingroup themeable
+ */
+#}
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
+  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="{{ language.langcode }}" lang="{{ language.langcode }}" dir="{{ language.dir }}">
+<head>
+  <title>{{ head_title }}</title>
+  {{ head }}
+  {{ styles }}
+  {{ scripts }}
+</head>
+<body class="{{ attributes.class }}">
+  <div id="page">
+    <div id="header">
+      <div id="logo-title">
+        {% if logo %}
+          <a href="{{ base_path }}" title="{{ 'Home'|t }}" rel="home" id="logo">
+            <img src="{{ logo }}" alt="{{ 'Home'|t }}" />
+          </a>
+        {% endif %}
+
+        <div id="name-and-slogan">
+          {% if site_name %}
+            <h1 id="site-name">
+              <a href="{{ base_path }}" title="{{ 'Home'|t }}" rel="home"><span>{{ site_name }}</span></a>
+            </h1>
+          {% endif %}
+          {% if site_slogan %}
+            <div id="site-slogan">{{ site_slogan }}</div>
+          {% endif %}
+        </div> <!-- /name-and-slogan -->
+      </div> <!-- /logo-title -->
+
+      {% if header %}
+        <div id="header-region">
+          {{ header }}
+        </div>
+      {% endif %}
+
+    </div> <!-- /header -->
+
+    <div id="container" class="clearfix">
+
+      {% if sidebar_first %}
+        <div id="sidebar-first" class="column sidebar">
+          {{ sidebar_first }}
+        </div> <!-- /sidebar-first -->
+      {% endif %}
+
+      <div id="main" class="column"><div id="main-squeeze">
+
+        <div id="content">
+          {% if title %}<h1 class="title" id="page-title">{{ title }}</h1>{% endif %}
+          {% if messages %}{{ messages }}{% endif %}
+          <div id="content-content" class="clearfix">
+            {{ content }}
+          </div> <!-- /content-content -->
+        </div> <!-- /content -->
+
+      </div></div> <!-- /main-squeeze /main -->
+
+      {% if sidebar_second %}
+        <div id="sidebar-second" class="column sidebar">
+          {{ sidebar_second }}
+        </div> <!-- /sidebar-second -->
+      {% endif %}
+
+    </div> <!-- /container -->
+
+    <div id="footer-wrapper">
+      <div id="footer">
+        {% if footer %}{{ footer }}{% endif %}
+      </div> <!-- /footer -->
+    </div> <!-- /footer-wrapper -->
+
+  </div> <!-- /page -->
+
+</body>
+</html>
diff --git a/core/modules/system/templates/maintenance-page.tpl.php b/core/modules/system/templates/maintenance-page.tpl.php
deleted file mode 100644
index 5da7a1a..0000000
--- a/core/modules/system/templates/maintenance-page.tpl.php
+++ /dev/null
@@ -1,95 +0,0 @@
-<?php
-
-/**
- * @file
- * Default theme implementation to display a single Drupal page while offline.
- *
- * All the available variables are mirrored in html.tpl.php and page.tpl.php.
- * Some may be blank but they are provided for consistency.
- *
- * @see template_preprocess()
- * @see template_preprocess_maintenance_page()
- *
- * @ingroup themeable
- */
-?>
-<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
-  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
-<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="<?php print $language->langcode ?>" lang="<?php print $language->langcode ?>" dir="<?php print $language->dir ?>">
-
-<head>
-  <title><?php print $head_title; ?></title>
-  <?php print $head; ?>
-  <?php print $styles; ?>
-  <?php print $scripts; ?>
-</head>
-<body class="<?php print $attributes['class']; ?>">
-  <div id="page">
-    <div id="header">
-      <div id="logo-title">
-
-        <?php if (!empty($logo)): ?>
-          <a href="<?php print $base_path; ?>" title="<?php print t('Home'); ?>" rel="home" id="logo">
-            <img src="<?php print $logo; ?>" alt="<?php print t('Home'); ?>" />
-          </a>
-        <?php endif; ?>
-
-        <div id="name-and-slogan">
-          <?php if (!empty($site_name)): ?>
-            <h1 id="site-name">
-              <a href="<?php print $base_path ?>" title="<?php print t('Home'); ?>" rel="home"><span><?php print $site_name; ?></span></a>
-            </h1>
-          <?php endif; ?>
-
-          <?php if (!empty($site_slogan)): ?>
-            <div id="site-slogan"><?php print $site_slogan; ?></div>
-          <?php endif; ?>
-        </div> <!-- /name-and-slogan -->
-      </div> <!-- /logo-title -->
-
-      <?php if (!empty($header)): ?>
-        <div id="header-region">
-          <?php print $header; ?>
-        </div>
-      <?php endif; ?>
-
-    </div> <!-- /header -->
-
-    <div id="container" class="clearfix">
-
-      <?php if (!empty($sidebar_first)): ?>
-        <div id="sidebar-first" class="column sidebar">
-          <?php print $sidebar_first; ?>
-        </div> <!-- /sidebar-first -->
-      <?php endif; ?>
-
-      <div id="main" class="column"><div id="main-squeeze">
-
-        <div id="content">
-          <?php if (!empty($title)): ?><h1 class="title" id="page-title"><?php print $title; ?></h1><?php endif; ?>
-          <?php if (!empty($messages)): print $messages; endif; ?>
-          <div id="content-content" class="clearfix">
-            <?php print $content; ?>
-          </div> <!-- /content-content -->
-        </div> <!-- /content -->
-
-      </div></div> <!-- /main-squeeze /main -->
-
-      <?php if (!empty($sidebar_second)): ?>
-        <div id="sidebar-second" class="column sidebar">
-          <?php print $sidebar_second; ?>
-        </div> <!-- /sidebar-second -->
-      <?php endif; ?>
-
-    </div> <!-- /container -->
-
-    <div id="footer-wrapper">
-      <div id="footer">
-        <?php if (!empty($footer)): print $footer; endif; ?>
-      </div> <!-- /footer -->
-    </div> <!-- /footer-wrapper -->
-
-  </div> <!-- /page -->
-
-</body>
-</html>
diff --git a/core/modules/system/templates/task-list.html.twig b/core/modules/system/templates/task-list.html.twig
new file mode 100644
index 0000000..64c3e77
--- /dev/null
+++ b/core/modules/system/templates/task-list.html.twig
@@ -0,0 +1,27 @@
+{#
+/**
+ * @file
+ * Default theme implementation for a list of maintenance tasks to perform.
+ *
+ * Available variables:
+ * - tasks: A list of maintenance tasks to perform. Each item in the list has
+ *   the following variables:
+ *   - item: The maintenance task.
+ *   - attributes: HTML attributes for the maintenance task.
+ *   - status: (optional) Text describing the status of the maintenance task,
+ *     'active' or 'done'.
+ *
+ * @see template_preprocess()
+ *
+ * @ingroup themeable
+ */
+#}
+<h2 class="element-invisible">Installation tasks</h2>
+<ol class="task-list">
+{% for task in tasks %}
+  <li class="{{ task.attributes.class }}">
+    {{- task.item -}}
+    {%- if task.status %} <span class="element-invisible">({{ task.status }})</span>{% endif -%}
+  </li>
+{% endfor %}
+</ol>