diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index f754ecb9268..681bdbbd8f5 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -34,7 +34,7 @@ function template_preprocess_menu_local_task(&$variables) {
   $link_text = $link['title'];
 
   if (!empty($variables['element']['#active'])) {
-    $variables['attributes']['class'] = array('active');
+    $variables['attributes']['class'] = array('is-active');
 
     // Add text to indicate active tab for non-visual users.
     $active = SafeMarkup::format('<span class="visually-hidden">@label</span>', array('@label' => t('(active tab)')));
diff --git a/core/includes/tablesort.inc b/core/includes/tablesort.inc
index 38cccc4af7d..65b791fc66c 100644
--- a/core/includes/tablesort.inc
+++ b/core/includes/tablesort.inc
@@ -48,7 +48,7 @@ function tablesort_header(&$cell_content, array &$cell_attributes, array $header
       // http://www.w3.org/TR/wai-aria/states_and_properties#aria-sort
       $cell_attributes['aria-sort'] = ($ts['sort'] == 'asc') ? 'ascending' : 'descending';
       $ts['sort'] = (($ts['sort'] == 'asc') ? 'desc' : 'asc');
-      $cell_attributes['class'][] = 'active';
+      $cell_attributes['class'][] = 'is-active';
       $tablesort_indicator = array(
         '#theme' => 'tablesort_indicator',
         '#style' => $ts['sort'],
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index dfee854c8a9..2aef832c819 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -549,7 +549,7 @@ function template_preprocess_datetime_wrapper(&$variables) {
  * list items differently when the link is active, since CSS does not yet allow
  * one to style list items only if it contains a certain element with a certain
  * class. I.e. we cannot yet convert this jQuery selector to a CSS selector:
- *   jQuery('li:has("a.active")')
+ *   jQuery('li:has("a.is-active")')
  *
  * @see \Drupal\Core\Utility\LinkGenerator
  * @see \Drupal\Core\Utility\LinkGenerator::generate()
@@ -1143,7 +1143,7 @@ function template_preprocess_maintenance_task_list(&$variables) {
     $variables['tasks'][$k]['item'] = $item;
     $variables['tasks'][$k]['attributes'] = new Attribute();
     if ($active == $k) {
-      $variables['tasks'][$k]['attributes']->addClass('active');
+      $variables['tasks'][$k]['attributes']->addClass('is-active');
       $variables['tasks'][$k]['status'] = t('active');
       $done = FALSE;
     }
diff --git a/core/lib/Drupal/Core/Menu/LocalTaskDefault.php b/core/lib/Drupal/Core/Menu/LocalTaskDefault.php
index 615eaa4ecc3..ecef19c5dae 100644
--- a/core/lib/Drupal/Core/Menu/LocalTaskDefault.php
+++ b/core/lib/Drupal/Core/Menu/LocalTaskDefault.php
@@ -113,8 +113,8 @@ class LocalTaskDefault extends PluginBase implements LocalTaskInterface {
   public function getOptions(RouteMatchInterface $route_match) {
     $options = $this->pluginDefinition['options'];
     if ($this->active) {
-      if (empty($options['attributes']['class']) || !in_array('active', $options['attributes']['class'])) {
-        $options['attributes']['class'][] = 'active';
+      if (empty($options['attributes']['class']) || !in_array('is-active', $options['attributes']['class'])) {
+        $options['attributes']['class'][] = 'is-active';
       }
     }
     return (array) $options;
diff --git a/core/misc/active-link.js b/core/misc/active-link.js
index 5bd12eba88d..c33c0ce6f68 100644
--- a/core/misc/active-link.js
+++ b/core/misc/active-link.js
@@ -8,7 +8,7 @@
   "use strict";
 
   /**
-   * Append active class.
+   * Append is-active class.
    *
    * The link is only active if its path corresponds to the current path, the
    * language of the linked path is equal to the current language, and if the
@@ -16,8 +16,8 @@
    * same request with different query parameters may yield a different page
    * (e.g. pagers, exposed View filters).
    *
-   * Does not discriminate based on element type, so allows you to set the active
-   * class on any element: a, li…
+   * Does not discriminate based on element type, so allows you to set the
+   * is-active class on any element: a, li…
    */
   Drupal.behaviors.activeLinks = {
     attach: function (context) {
@@ -47,14 +47,14 @@
       // Query the DOM.
       var activeLinks = context.querySelectorAll(selectors.join(','));
       for (var i = 0, il = activeLinks.length; i < il; i += 1) {
-        activeLinks[i].classList.add('active');
+        activeLinks[i].classList.add('is-active');
       }
     },
     detach: function (context, settings, trigger) {
       if (trigger === 'unload') {
-        var activeLinks = context.querySelectorAll('[data-drupal-link-system-path].active');
+        var activeLinks = context.querySelectorAll('[data-drupal-link-system-path].is-active');
         for (var i = 0, il = activeLinks.length; i < il; i += 1) {
-          activeLinks[i].classList.remove('active');
+          activeLinks[i].classList.remove('is-active');
         }
       }
     }
diff --git a/core/modules/contextual/css/contextual.icons.theme.css b/core/modules/contextual/css/contextual.icons.theme.css
index 70363a78885..7eb3f43eeb6 100644
--- a/core/modules/contextual/css/contextual.icons.theme.css
+++ b/core/modules/contextual/css/contextual.icons.theme.css
@@ -10,7 +10,7 @@
   background-image: url(../../../misc/icons/bebebe/pencil.svg);
 }
 .toolbar-bar .toolbar-icon-edit:active:before,
-.toolbar-bar .toolbar-icon-edit.active:before {
+.toolbar-bar .toolbar-icon-edit.is-active:before {
   background-image: url(../../../misc/icons/ffffff/pencil.svg);
 }
 
diff --git a/core/modules/contextual/css/contextual.toolbar.css b/core/modules/contextual/css/contextual.toolbar.css
index bda844c2664..3c82a1503a4 100644
--- a/core/modules/contextual/css/contextual.toolbar.css
+++ b/core/modules/contextual/css/contextual.toolbar.css
@@ -19,7 +19,7 @@
 [dir="rtl"] .toolbar .toolbar-bar .contextual-toolbar-tab .toolbar-item {
   padding-right: 1.3333em;
 }
-.toolbar .toolbar-bar .contextual-toolbar-tab .toolbar-item.active {
+.toolbar .toolbar-bar .contextual-toolbar-tab .toolbar-item.is-active {
   background-image:-webkit-linear-gradient(rgb(78,159,234) 0%, rgb(69,132,221) 100%);
   background-image:linear-gradient(rgb(78,159,234) 0%,rgb(69,132,221) 100%);
 }
diff --git a/core/modules/contextual/js/toolbar/views/VisualView.js b/core/modules/contextual/js/toolbar/views/VisualView.js
index 9b444517c8b..41b095f36b5 100644
--- a/core/modules/contextual/js/toolbar/views/VisualView.js
+++ b/core/modules/contextual/js/toolbar/views/VisualView.js
@@ -44,7 +44,7 @@
       // Render the visibility.
       this.$el.toggleClass('hidden', !this.model.get('isVisible'));
       // Render the state.
-      this.$el.find('button').toggleClass('active', !this.model.get('isViewing'));
+      this.$el.find('button').toggleClass('is-active', !this.model.get('isViewing'));
 
       return this;
     },
diff --git a/core/modules/language/src/Tests/LanguageSwitchingTest.php b/core/modules/language/src/Tests/LanguageSwitchingTest.php
index 1c39a8854ac..c9ff8d06638 100644
--- a/core/modules/language/src/Tests/LanguageSwitchingTest.php
+++ b/core/modules/language/src/Tests/LanguageSwitchingTest.php
@@ -140,14 +140,14 @@ class LanguageSwitchingTest extends WebTestBase {
     foreach ($language_switcher->ul->li as $link) {
       $classes = explode(" ", (string) $link['class']);
       list($langcode) = array_intersect($classes, array('en', 'fr'));
-      if (in_array('active', $classes)) {
+      if (in_array('is-active', $classes)) {
         $links['active'][] = $langcode;
       }
       else {
         $links['inactive'][] = $langcode;
       }
       $anchor_classes = explode(" ", (string) $link->a['class']);
-      if (in_array('active', $anchor_classes)) {
+      if (in_array('is-active', $anchor_classes)) {
         $anchors['active'][] = $langcode;
       }
       else {
@@ -359,17 +359,17 @@ class LanguageSwitchingTest extends WebTestBase {
 
     // Language code 'none' link should be active.
     $langcode = 'none';
-    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'no_lang_link', ':class' => 'active'));
+    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'no_lang_link', ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), t('A link generated by :function to the current :language page with langcode :langcode is marked active.', array(':function' => $function_name, ':language' => $current_language, ':langcode' => $langcode)));
 
     // Language code 'en' link should be active.
     $langcode = 'en';
-    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'en_link', ':class' => 'active'));
+    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'en_link', ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), t('A link generated by :function to the current :language page with langcode :langcode is marked active.', array(':function' => $function_name, ':language' => $current_language, ':langcode' => $langcode)));
 
     // Language code 'fr' link should not be active.
     $langcode = 'fr';
-    $links = $this->xpath('//a[@id = :id and not(contains(@class, :class))]', array(':id' => 'fr_link', ':class' => 'active'));
+    $links = $this->xpath('//a[@id = :id and not(contains(@class, :class))]', array(':id' => 'fr_link', ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), t('A link generated by :function to the current :language page with langcode :langcode is NOT marked active.', array(':function' => $function_name, ':language' => $current_language, ':langcode' => $langcode)));
 
     // Test links generated by _l() on a French page.
@@ -378,17 +378,17 @@ class LanguageSwitchingTest extends WebTestBase {
 
     // Language code 'none' link should be active.
     $langcode = 'none';
-    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'no_lang_link', ':class' => 'active'));
+    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'no_lang_link', ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), t('A link generated by :function to the current :language page with langcode :langcode is marked active.', array(':function' => $function_name, ':language' => $current_language, ':langcode' => $langcode)));
 
     // Language code 'en' link should not be active.
     $langcode = 'en';
-    $links = $this->xpath('//a[@id = :id and not(contains(@class, :class))]', array(':id' => 'en_link', ':class' => 'active'));
+    $links = $this->xpath('//a[@id = :id and not(contains(@class, :class))]', array(':id' => 'en_link', ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), t('A link generated by :function to the current :language page with langcode :langcode is NOT marked active.', array(':function' => $function_name, ':language' => $current_language, ':langcode' => $langcode)));
 
     // Language code 'fr' link should be active.
     $langcode = 'fr';
-    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'fr_link', ':class' => 'active'));
+    $links = $this->xpath('//a[@id = :id and contains(@class, :class)]', array(':id' => 'fr_link', ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), t('A link generated by :function to the current :language page with langcode :langcode is marked active.', array(':function' => $function_name, ':language' => $current_language, ':langcode' => $langcode)));
   }
 
diff --git a/core/modules/language/src/Tests/LanguageUILanguageNegotiationTest.php b/core/modules/language/src/Tests/LanguageUILanguageNegotiationTest.php
index 44bfdabb7c1..5cc97131a3c 100644
--- a/core/modules/language/src/Tests/LanguageUILanguageNegotiationTest.php
+++ b/core/modules/language/src/Tests/LanguageUILanguageNegotiationTest.php
@@ -402,7 +402,7 @@ class LanguageUILanguageNegotiationTest extends WebTestBase {
     // Check that the language switcher active link matches the given browser
     // language.
     $args = array(':id' => 'block-test-language-block', ':url' => \Drupal::url('<front>') . $langcode_browser_fallback);
-    $fields = $this->xpath('//div[@id=:id]//a[@class="language-link active" and starts-with(@href, :url)]', $args);
+    $fields = $this->xpath('//div[@id=:id]//a[@class="language-link is-active" and starts-with(@href, :url)]', $args);
     $this->assertTrue($fields[0] == $languages[$langcode_browser_fallback]->getName(), 'The browser language is the URL active language');
 
     // Check that URLs are rewritten using the given browser language.
diff --git a/core/modules/shortcut/css/shortcut.icons.theme.css b/core/modules/shortcut/css/shortcut.icons.theme.css
index e15f7d8141d..54569c2e48d 100644
--- a/core/modules/shortcut/css/shortcut.icons.theme.css
+++ b/core/modules/shortcut/css/shortcut.icons.theme.css
@@ -10,7 +10,7 @@
   background-image: url(../../../misc/icons/bebebe/star.svg);
 }
 .toolbar-bar .toolbar-icon-shortcut:active:before,
-.toolbar-bar .toolbar-icon-shortcut.active:before {
+.toolbar-bar .toolbar-icon-shortcut.is-active:before {
   background-image: url(../../../misc/icons/ffffff/star.svg);
 }
 
diff --git a/core/modules/system/css/system.theme.css b/core/modules/system/css/system.theme.css
index f7af1e4eb04..0f5b08fdd16 100644
--- a/core/modules/system/css/system.theme.css
+++ b/core/modules/system/css/system.theme.css
@@ -13,10 +13,10 @@
 /**
  * Markup generated by tablesort-indicator.html.twig.
  */
-th.active img {
+th.is-active img {
   display: inline;
 }
-td.active {
+td.is-active {
   background-color: #ddd;
 }
 
@@ -361,7 +361,7 @@ ul.menu {
   padding-top: 0.2em;
   margin: 0;
 }
-ul.menu a.active {
+ul.menu a.is-active {
   color: #000;
 }
 
@@ -378,7 +378,7 @@ ul.inline li {
   list-style-type: none;
   padding: 0 0.5em;
 }
-ul.links a.active {
+ul.links a.is-active {
   color: #000;
 }
 
@@ -434,7 +434,7 @@ ul.tabs {
   padding: 0.2em 1em;
   text-decoration: none;
 }
-.tabs a.active {
+.tabs a.is-active {
   background-color: #eee;
 }
 .tabs a:focus,
diff --git a/core/modules/system/src/Controller/SystemController.php b/core/modules/system/src/Controller/SystemController.php
index 73180a08761..90c260cf28a 100644
--- a/core/modules/system/src/Controller/SystemController.php
+++ b/core/modules/system/src/Controller/SystemController.php
@@ -329,7 +329,7 @@ class SystemController extends ControllerBase {
   }
 
   /**
-   * #post_render_cache callback; sets the "active" class on relevant links.
+   * #post_render_cache callback; sets the "is-active" class on relevant links.
    *
    * This is a PHP implementation of the drupal.active-link JavaScript library.
    *
@@ -405,7 +405,7 @@ class SystemController extends ControllerBase {
 
       // Ensure we don't set the "active" class twice on the same element.
       $class = $node->getAttribute('class');
-      $add_active = !in_array('active', explode(' ', $class));
+      $add_active = !in_array('is-active', explode(' ', $class));
 
       // The language of an active link is equal to the current language.
       if ($add_active && $context['language']) {
@@ -429,12 +429,12 @@ class SystemController extends ControllerBase {
       }
 
       // Only if the path, the language and the query match, we set the
-      // "active" class.
+      // "is-active" class.
       if ($add_active) {
         if (strlen($class) > 0) {
           $class .= ' ';
         }
-        $class .= 'active';
+        $class .= 'is-active';
         $node->setAttribute('class', $class);
 
         // Get the updated tag.
diff --git a/core/modules/system/src/Tests/Common/UrlTest.php b/core/modules/system/src/Tests/Common/UrlTest.php
index ad97b93efe0..38641c1edd1 100644
--- a/core/modules/system/src/Tests/Common/UrlTest.php
+++ b/core/modules/system/src/Tests/Common/UrlTest.php
@@ -87,20 +87,20 @@ class UrlTest extends WebTestBase {
     $path = 'common-test/type-link-active-class';
 
     $this->drupalGet($path, $options_no_query);
-    $links = $this->xpath('//a[@href = :href and contains(@class, :class)]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_no_query)->toString(), ':class' => 'active'));
+    $links = $this->xpath('//a[@href = :href and contains(@class, :class)]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_no_query)->toString(), ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), 'A link generated by _l() to the current page is marked active.');
 
-    $links = $this->xpath('//a[@href = :href and not(contains(@class, :class))]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_query)->toString(), ':class' => 'active'));
+    $links = $this->xpath('//a[@href = :href and not(contains(@class, :class))]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_query)->toString(), ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), 'A link generated by _l() to the current page with a query string when the current page has no query string is not marked active.');
 
     $this->drupalGet($path, $options_query);
-    $links = $this->xpath('//a[@href = :href and contains(@class, :class)]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_query)->toString(), ':class' => 'active'));
+    $links = $this->xpath('//a[@href = :href and contains(@class, :class)]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_query)->toString(), ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), 'A link generated by _l() to the current page with a query string that matches the current query string is marked active.');
 
-    $links = $this->xpath('//a[@href = :href and contains(@class, :class)]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_query_reverse)->toString(), ':class' => 'active'));
+    $links = $this->xpath('//a[@href = :href and contains(@class, :class)]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_query_reverse)->toString(), ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), 'A link generated by _l() to the current page with a query string that has matching parameters to the current query string but in a different order is marked active.');
 
-    $links = $this->xpath('//a[@href = :href and not(contains(@class, :class))]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_no_query)->toString(), ':class' => 'active'));
+    $links = $this->xpath('//a[@href = :href and not(contains(@class, :class))]', array(':href' => Url::fromRoute('common_test.l_active_class', [], $options_no_query)->toString(), ':class' => 'is-active'));
     $this->assertTrue(isset($links[0]), 'A link generated by _l() to the current page without a query string when the current page has a query string is not marked active.');
 
     // Test adding a custom class in links produced by _l() and #type 'link'.
diff --git a/core/modules/system/src/Tests/Menu/AssertMenuActiveTrailTrait.php b/core/modules/system/src/Tests/Menu/AssertMenuActiveTrailTrait.php
index fba46d470ad..125246ca207 100644
--- a/core/modules/system/src/Tests/Menu/AssertMenuActiveTrailTrait.php
+++ b/core/modules/system/src/Tests/Menu/AssertMenuActiveTrailTrait.php
@@ -56,7 +56,7 @@ trait AssertMenuActiveTrailTrait {
     $xpath .= 'li[contains(@class, :class-trail)]/a[contains(@href, :href) ' . $xpath_last_active . 'and contains(text(), :title)]';
     $args = array(
       ':class-trail' => 'menu-item--active-trail',
-      ':class-active' => 'active',
+      ':class-active' => 'is-active',
       ':href' => Url::fromUri('base:' . $active_link_path)->toString(),
       ':title' => $active_link_title,
     );
diff --git a/core/modules/system/tests/src/Unit/Controller/SystemControllerTest.php b/core/modules/system/tests/src/Unit/Controller/SystemControllerTest.php
index c719a6ab7c4..173a4300929 100644
--- a/core/modules/system/tests/src/Unit/Controller/SystemControllerTest.php
+++ b/core/modules/system/tests/src/Unit/Controller/SystemControllerTest.php
@@ -25,10 +25,10 @@ class SystemControllerTest extends UnitTestCase {
    * @see \Drupal\system\Controller\SystemController::setLinkActiveClass()
    */
   public function providerTestSetLinkActiveClass() {
-    // Define all the variations that *don't* affect whether or not an "active"
-    // class is set, but that should remain unchanged:
+    // Define all the variations that *don't* affect whether or not an
+    // "is-active" class is set, but that should remain unchanged:
     // - surrounding HTML
-    // - tags for which to test the setting of the "active" class
+    // - tags for which to test the setting of the "is-active" class
     // - content of said tags
     $edge_case_html5 = '<audio src="foo.ogg">
   <track kind="captions" src="foo.en.vtt" srclang="en" label="English">
@@ -40,7 +40,7 @@ class SystemControllerTest extends UnitTestCase {
       // Tricky HTML5 example that's unsupported by PHP <=5.4's DOMDocument:
       // https://drupal.org/comment/7938201#comment-7938201.
       1 => array('prefix' => '<div><p>', 'suffix' => '</p>' . $edge_case_html5 . '</div>'),
-      // Multi-byte content *before* the HTML that needs the "active" class.
+      // Multi-byte content *before* the HTML that needs the "is-active" class.
       2 => array('prefix' => '<div><p>αβγδεζηθικλμνξοσὠ</p><p>', 'suffix' => '</p></div>'),
     );
     $tags = array(
@@ -63,8 +63,8 @@ class SystemControllerTest extends UnitTestCase {
       'data-drupal-link-system-path=&quot;&lt;front&gt;&quot;',
     );
 
-    // Define all variations that *do* affect whether or not an "active" class
-    // is set: all possible situations that can be encountered.
+    // Define all variations that *do* affect whether or not an "is-active"
+    // class is set: all possible situations that can be encountered.
     $situations = array();
 
     // Situations with context: front page, English, no query.
@@ -275,10 +275,10 @@ class SystemControllerTest extends UnitTestCase {
             // Build the source markup.
             $source_markup = $create_markup(new Attribute($situation['attributes']));
 
-            // Build the target markup. If no "active" class should be set, the
-            // resulting HTML should be identical. Otherwise, it should get an
-            // "active" class, either by extending an existing "class" attribute
-            // or by adding a "class" attribute.
+            // Build the target markup. If no "is-active" class should be set,
+            // the resulting HTML should be identical. Otherwise, it should get
+            // an "is-active" class, either by extending an existing "class"
+            // attribute or by adding a "class" attribute.
             $target_markup = NULL;
             if (!$situation['is active']) {
               $target_markup = $source_markup;
@@ -288,7 +288,7 @@ class SystemControllerTest extends UnitTestCase {
               if (!isset($active_attributes['class'])) {
                 $active_attributes['class'] = array();
               }
-              $active_attributes['class'][] = 'active';
+              $active_attributes['class'][] = 'is-active';
               $target_markup = $create_markup(new Attribute($active_attributes));
             }
 
@@ -298,21 +298,23 @@ class SystemControllerTest extends UnitTestCase {
       }
     }
 
-    // Test case to verify that the 'active' class is not added multiple times.
+    // Test case to verify that the 'is-active' class is not added multiple
+    // times.
     $data[] = [
       0 => ['#markup' => '<a data-drupal-link-system-path="&lt;front&gt;">Once</a> <a data-drupal-link-system-path="&lt;front&gt;">Twice</a>'],
       1 => ['path' => '', 'front' => TRUE, 'language' => 'en', 'query' => []],
-      2 => ['#markup' => '<a data-drupal-link-system-path="&lt;front&gt;" class="active">Once</a> <a data-drupal-link-system-path="&lt;front&gt;" class="active">Twice</a>'],
+      2 => ['#markup' => '<a data-drupal-link-system-path="&lt;front&gt;" class="is-active">Once</a> <a data-drupal-link-system-path="&lt;front&gt;" class="is-active">Twice</a>'],
     ];
 
-    // Test cases to verify that the 'active' class is added when on the front
-    // page, and there are two different kinds of matching links on the page:
+    // Test cases to verify that the 'is-active' class is added when on the
+    // front page, and there are two different kinds of matching links on the
+    // page:
     // - the matching path (the resolved front page path)
     // - the special matching path ('<front>')
     $front_special_link = '<a data-drupal-link-system-path="&lt;front&gt;">Front</a>';
-    $front_special_link_active = '<a data-drupal-link-system-path="&lt;front&gt;" class="active">Front</a>';
+    $front_special_link_active = '<a data-drupal-link-system-path="&lt;front&gt;" class="is-active">Front</a>';
     $front_path_link = '<a data-drupal-link-system-path="myfrontpage">Front Path</a>';
-    $front_path_link_active = '<a data-drupal-link-system-path="myfrontpage" class="active">Front Path</a>';
+    $front_path_link_active = '<a data-drupal-link-system-path="myfrontpage" class="is-active">Front Path</a>';
     $data[] = [
       0 => ['#markup' => $front_path_link . ' ' . $front_special_link],
       1 => ['path' => 'myfrontpage', 'front' => TRUE, 'language' => 'en', 'query' => []],
diff --git a/core/modules/toolbar/css/toolbar.icons.theme.css b/core/modules/toolbar/css/toolbar.icons.theme.css
index 3f274f37fa8..3a5549e12b8 100644
--- a/core/modules/toolbar/css/toolbar.icons.theme.css
+++ b/core/modules/toolbar/css/toolbar.icons.theme.css
@@ -75,14 +75,14 @@
   background-image: url(../../../misc/icons/bebebe/hamburger.svg);
 }
 .toolbar-bar .toolbar-icon-menu:active:before,
-.toolbar-bar .toolbar-icon-menu.active:before {
+.toolbar-bar .toolbar-icon-menu.is-active:before {
   background-image: url(../../../misc/icons/ffffff/hamburger.svg);
 }
 .toolbar-bar .toolbar-icon-help:before {
   background-image: url(../../../misc/icons/bebebe/questionmark-disc.svg);
 }
 .toolbar-bar .toolbar-icon-help:active:before,
-.toolbar-bar .toolbar-icon-help.active:before {
+.toolbar-bar .toolbar-icon-help.is-active:before {
   background-image: url(../../../misc/icons/ffffff/questionmark-disc.svg);
 }
 
@@ -93,56 +93,56 @@
   background-image: url(../../../misc/icons/787878/file.svg);
 }
 .toolbar-icon-system-admin-content:active:before,
-.toolbar-icon-system-admin-content.active:before {
+.toolbar-icon-system-admin-content.is-active:before {
   background-image: url(../../../misc/icons/000000/file.svg);
 }
 .toolbar-icon-system-admin-structure:before {
   background-image: url(../../../misc/icons/787878/orgchart.svg);
 }
 .toolbar-icon-system-admin-structure:active:before,
-.toolbar-icon-system-admin-structure.active:before {
+.toolbar-icon-system-admin-structure.is-active:before {
   background-image: url(../../../misc/icons/000000/orgchart.svg);
 }
 .toolbar-icon-system-themes-page:before {
   background-image: url(../../../misc/icons/787878/paintbrush.svg);
 }
 .toolbar-icon-system-themes-page:active:before,
-.toolbar-icon-system-themes-page.active:before {
+.toolbar-icon-system-themes-page.is-active:before {
   background-image: url(../../../misc/icons/000000/paintbrush.svg);
 }
 .toolbar-icon-entity-user-collection:before {
   background-image: url(../../../misc/icons/787878/people.svg);
 }
 .toolbar-icon-entity-user-collection:active:before,
-.toolbar-icon-entity-user-collection.active:before {
+.toolbar-icon-entity-user-collection.is-active:before {
   background-image: url(../../../misc/icons/000000/people.svg);
 }
 .toolbar-icon-system-modules-list:before {
   background-image: url(../../../misc/icons/787878/puzzlepiece.svg);
 }
 .toolbar-icon-system-modules-list:active:before,
-.toolbar-icon-system-modules-list.active:before {
+.toolbar-icon-system-modules-list.is-active:before {
   background-image: url(../../../misc/icons/000000/puzzlepiece.svg);
 }
 .toolbar-icon-system-admin-config:before {
   background-image: url(../../../misc/icons/787878/wrench.svg);
 }
 .toolbar-icon-system-admin-config:active:before,
-.toolbar-icon-system-admin-config.active:before {
+.toolbar-icon-system-admin-config.is-active:before {
   background-image: url(../../../misc/icons/000000/wrench.svg);
 }
 .toolbar-icon-system-admin-reports:before {
   background-image: url(../../../misc/icons/787878/barchart.svg);
 }
 .toolbar-icon-system-admin-reports:active:before,
-.toolbar-icon-system-admin-reports.active:before {
+.toolbar-icon-system-admin-reports.is-active:before {
   background-image: url(../../../misc/icons/000000/barchart.svg);
 }
 .toolbar-icon-help-main:before {
   background-image: url(../../../misc/icons/787878/questionmark-disc.svg);
 }
 .toolbar-icon-help-main:active:before,
-.toolbar-icon-help-main.active:before {
+.toolbar-icon-help-main.is-active:before {
   background-image: url(../../../misc/icons/000000/questionmark-disc.svg);
 }
 .toolbar .toolbar-bar .toolbar-icon:before {
diff --git a/core/modules/toolbar/css/toolbar.menu.css b/core/modules/toolbar/css/toolbar.menu.css
index e857beb441e..14dbd44c575 100644
--- a/core/modules/toolbar/css/toolbar.menu.css
+++ b/core/modules/toolbar/css/toolbar.menu.css
@@ -29,14 +29,14 @@
   margin-right: 0;
 }
 .toolbar .toolbar-tray .menu-item--active-trail > .toolbar-box a,
-.toolbar .toolbar-tray a.active {
+.toolbar .toolbar-tray a.is-active {
   color: #000;
   font-weight: bold;
 }
 
 /* ----- Toolbar menu tray for viewports less than 320px ------ */
 @media screen and (max-width: 319px) {
-  .toolbar .toolbar-tray-vertical.active {
+  .toolbar .toolbar-tray-vertical.is-active {
     width: 100%;
   }
 }
diff --git a/core/modules/toolbar/css/toolbar.module.css b/core/modules/toolbar/css/toolbar.module.css
index 2ab2e4d532b..771b7050611 100644
--- a/core/modules/toolbar/css/toolbar.module.css
+++ b/core/modules/toolbar/css/toolbar.module.css
@@ -183,21 +183,21 @@ body.toolbar-fixed .toolbar .toolbar-tray-horizontal {
 /* When the configured standard breakpoint is active and the tray is in a
  * vertical position, the tray does not scroll with the page. The contents of
  * the tray scroll within the confines of the viewport. */
-.toolbar .toolbar-tray-vertical.active,
+.toolbar .toolbar-tray-vertical.is-active,
 body.toolbar-fixed .toolbar .toolbar-tray-vertical {
   height: 100%;
   overflow-x: hidden;
   overflow-y: auto;
   position: fixed;
 }
-.toolbar .toolbar-tray.active {
+.toolbar .toolbar-tray.is-active {
   display: block;
 }
 /* Bring the tray into the viewport. By default it is just off-screen. */
-.toolbar-oriented .toolbar-tray-vertical.active {
+.toolbar-oriented .toolbar-tray-vertical.is-active {
   left: 0; /* LTR */
 }
-[dir="rtl"] .toolbar-oriented .toolbar-tray-vertical.active {
+[dir="rtl"] .toolbar-oriented .toolbar-tray-vertical.is-active {
   left: auto;
   right: 0;
 }
diff --git a/core/modules/toolbar/css/toolbar.theme.css b/core/modules/toolbar/css/toolbar.theme.css
index 8946e5f53d2..21aed489e3b 100644
--- a/core/modules/toolbar/css/toolbar.theme.css
+++ b/core/modules/toolbar/css/toolbar.theme.css
@@ -47,7 +47,7 @@
   background-image: linear-gradient(rgba(255, 255, 255, 0.125) 20%, transparent 200%);
   text-decoration: none;
 }
-.toolbar .toolbar-bar .toolbar-tab > .toolbar-item.active {
+.toolbar .toolbar-bar .toolbar-tab > .toolbar-item.is-active {
   background-image: -webkit-linear-gradient(rgba(255, 255, 255, 0.25) 20%, transparent 200%);
   background-image: linear-gradient(rgba(255, 255, 255, 0.25) 20%, transparent 200%);
 }
@@ -94,7 +94,7 @@
 .toolbar-tray a:hover,
 .toolbar-tray a:active,
 .toolbar-tray a:focus,
-.toolbar-tray a.active
+.toolbar-tray a.is-active
  {
   color: #000;
   text-decoration: underline;
diff --git a/core/modules/toolbar/js/views/ToolbarVisualView.js b/core/modules/toolbar/js/views/ToolbarVisualView.js
index aaccf172b3d..f30bde3276f 100644
--- a/core/modules/toolbar/js/views/ToolbarVisualView.js
+++ b/core/modules/toolbar/js/views/ToolbarVisualView.js
@@ -135,16 +135,16 @@
       var $tab = $(this.model.get('activeTab'));
       // Deactivate the previous tab.
       $(this.model.previous('activeTab'))
-        .removeClass('active')
+        .removeClass('is-active')
         .prop('aria-pressed', false);
       // Deactivate the previous tray.
       $(this.model.previous('activeTray'))
-        .removeClass('active');
+        .removeClass('is-active');
 
       // Activate the selected tab.
       if ($tab.length > 0) {
         $tab
-          .addClass('active')
+          .addClass('is-active')
           // Mark the tab as pressed.
           .prop('aria-pressed', true);
         var name = $tab.attr('data-toolbar-tray');
@@ -156,7 +156,7 @@
         // Activate the associated tray.
         var $tray = this.$el.find('[data-toolbar-tray="' + name + '"].toolbar-tray');
         if ($tray.length) {
-          $tray.addClass('active');
+          $tray.addClass('is-active');
           this.model.set('activeTray', $tray.get(0));
         }
         else {
@@ -218,9 +218,9 @@
       // Remove data-offset attributes from the trays so they can be refreshed.
       $trays.removeAttr('data-offset-left data-offset-right data-offset-top');
       // If an active vertical tray exists, mark it as an offset element.
-      $trays.filter('.toolbar-tray-vertical.active').attr('data-offset-' + edge, '');
+      $trays.filter('.toolbar-tray-vertical.is-active').attr('data-offset-' + edge, '');
       // If an active horizontal tray exists, mark it as an offset element.
-      $trays.filter('.toolbar-tray-horizontal.active').attr('data-offset-top', '');
+      $trays.filter('.toolbar-tray-horizontal.is-active').attr('data-offset-top', '');
     },
 
     /**
diff --git a/core/modules/tour/css/tour.module.css b/core/modules/tour/css/tour.module.css
index 6112b48f0ba..2d1f4c8b3f7 100644
--- a/core/modules/tour/css/tour.module.css
+++ b/core/modules/tour/css/tour.module.css
@@ -16,7 +16,7 @@
   color: #fff;
   font-weight: bold;
 }
-.toolbar .tour-toolbar-tab button.active {
+.toolbar .tour-toolbar-tab button.is-active {
   background-image: -webkit-linear-gradient(rgba(255, 255, 255, 0.25) 20%, transparent 200%);
   background-image: linear-gradient(rgba(255, 255, 255, 0.25) 20%, transparent 200%);
 }
diff --git a/core/modules/tour/js/tour.js b/core/modules/tour/js/tour.js
index 8d85e105cb6..c2c7ac375f5 100644
--- a/core/modules/tour/js/tour.js
+++ b/core/modules/tour/js/tour.js
@@ -86,7 +86,7 @@
       // Render the state.
       var isActive = this.model.get('isActive');
       this.$el.find('button')
-        .toggleClass('active', isActive)
+        .toggleClass('is-active', isActive)
         .prop('aria-pressed', isActive);
       return this;
     },
diff --git a/core/modules/user/css/user.icons.theme.css b/core/modules/user/css/user.icons.theme.css
index 11ae8393624..66c2366c026 100644
--- a/core/modules/user/css/user.icons.theme.css
+++ b/core/modules/user/css/user.icons.theme.css
@@ -10,6 +10,6 @@
   background-image: url(../../../misc/icons/bebebe/person.svg);
 }
 .toolbar-bar .toolbar-icon-user:active:before,
-.toolbar-bar .toolbar-icon-user.active:before {
+.toolbar-bar .toolbar-icon-user.is-active:before {
   background-image: url(../../../misc/icons/ffffff/person.svg);
 }
diff --git a/core/modules/views/src/Tests/Plugin/DisplayPageWebTest.php b/core/modules/views/src/Tests/Plugin/DisplayPageWebTest.php
index 6eff94cf35f..2da04f34e11 100644
--- a/core/modules/views/src/Tests/Plugin/DisplayPageWebTest.php
+++ b/core/modules/views/src/Tests/Plugin/DisplayPageWebTest.php
@@ -87,7 +87,7 @@ class DisplayPageWebTest extends PluginTestBase {
     $this->assertResponse(200);
     $element = $this->xpath('//ul[contains(@class, :ul_class)]//a[contains(@class, :a_class)]', array(
       ':ul_class' => 'tabs primary',
-      ':a_class' => 'active',
+      ':a_class' => 'is-active',
     ));
     $this->assertEqual((string) $element[0], t('Test default tab'));
     $this->assertTitle(t('Test default page | Drupal'));
@@ -99,7 +99,7 @@ class DisplayPageWebTest extends PluginTestBase {
     $this->assertResponse(200);
     $element = $this->xpath('//ul[contains(@class, :ul_class)]//a[contains(@class, :a_class)]', array(
       ':ul_class' => 'tabs primary',
-      ':a_class' => 'active',
+      ':a_class' => 'is-active',
     ));
     $this->assertEqual((string) $element[0], t('Test local tab'));
     $this->assertTitle(t('Test local page | Drupal'));
diff --git a/core/modules/views/templates/views-view-summary-unformatted.html.twig b/core/modules/views/templates/views-view-summary-unformatted.html.twig
index 57a6e830f41..df90d4056e1 100644
--- a/core/modules/views/templates/views-view-summary-unformatted.html.twig
+++ b/core/modules/views/templates/views-view-summary-unformatted.html.twig
@@ -25,7 +25,7 @@
   {% if row.separator -%}
     {{ row.separator }}
   {%- endif %}
-  <a href="{{ row.url }}"{{ row.attributes.addClass(row.active ? 'active') }}>{{ row.link }}</a>
+  <a href="{{ row.url }}"{{ row.attributes.addClass(row.active ? 'is-active') }}>{{ row.link }}</a>
   {% if options.count %}
     ({{ row.count }})
   {% endif %}
diff --git a/core/modules/views/templates/views-view-summary.html.twig b/core/modules/views/templates/views-view-summary.html.twig
index f1e788bc69a..86b78ad4309 100644
--- a/core/modules/views/templates/views-view-summary.html.twig
+++ b/core/modules/views/templates/views-view-summary.html.twig
@@ -23,7 +23,7 @@
 <div class="item-list">
   <ul class="views-summary">
   {% for row in rows %}
-    <li><a href="{{ row.url }}"{{ row.attributes.addClass(row.active ? 'active') }}>{{ row.link }}</a>
+    <li><a href="{{ row.url }}"{{ row.attributes.addClass(row.active ? 'is-active') }}>{{ row.link }}</a>
       {% if options.count %}
         ({{ row.count }})
       {% endif %}
diff --git a/core/modules/views/views.theme.inc b/core/modules/views/views.theme.inc
old mode 100755
new mode 100644
index 07659f5f7d8..e4e56ab1892
--- a/core/modules/views/views.theme.inc
+++ b/core/modules/views/views.theme.inc
@@ -462,7 +462,7 @@ function template_preprocess_views_view_table(&$variables) {
     // what the CSS classes should be.
     $variables['fields'][$field] = Html::cleanCssIdentifier($field);
     if ($active == $field) {
-      $variables['fields'][$field] .= ' active';
+      $variables['fields'][$field] .= ' is-active';
     }
 
     // Render the header labels.
diff --git a/core/modules/views_ui/css/views_ui.admin.theme.css b/core/modules/views_ui/css/views_ui.admin.theme.css
index 4486d5b358c..57d461a863a 100644
--- a/core/modules/views_ui/css/views_ui.admin.theme.css
+++ b/core/modules/views_ui/css/views_ui.admin.theme.css
@@ -479,7 +479,7 @@ td.group-title {
 }
 
 .views-displays .tabs.secondary li,
-.views-displays .tabs.secondary li.active {
+.views-displays .tabs.secondary li.is-active {
   background: transparent;
   margin-bottom: 5px;
   border: 0;
@@ -524,7 +524,7 @@ td.group-title {
 /**
  * Display a red border if the display doesn't validate.
  */
-.views-displays .tabs.secondary li.active a.active.error,
+.views-displays .tabs.secondary li.is-active a.is-active.error,
 .views-displays .tabs.secondary a.error {
   border: 2px solid #ed541d;
   padding: 1px 6px;
@@ -539,8 +539,8 @@ td.group-title {
 }
 
 .views-displays .tabs.secondary li a:hover,
-.views-displays .tabs.secondary li.active a,
-.views-displays .tabs.secondary li.active a.active {
+.views-displays .tabs.secondary li.is-active a,
+.views-displays .tabs.secondary li.is-active a.is-active {
   background-color: #555;
   color: #fff;
 }
diff --git a/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php b/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
index 0141418cecf..573444b9bd8 100644
--- a/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
+++ b/core/tests/Drupal/Tests/Core/Menu/LocalTaskDefaultTest.php
@@ -295,7 +295,7 @@ class LocalTaskDefaultTest extends UnitTestCase {
       'attributes' => array(
         'class' => array(
           'example',
-          'active'
+          'is-active'
         )
       )
     ), $this->localTaskBase->getOptions($route_match));
diff --git a/core/themes/bartik/color/preview.css b/core/themes/bartik/color/preview.css
index 294277f561f..18af7f22a8c 100644
--- a/core/themes/bartik/color/preview.css
+++ b/core/themes/bartik/color/preview.css
@@ -61,7 +61,7 @@
   text-decoration: none;
   cursor: pointer;
 }
-#preview-main-menu-links li a.active {
+#preview-main-menu-links li a.is-active {
   background: #fff;
   border-bottom: none;
 }
diff --git a/core/themes/bartik/css/colors.css b/core/themes/bartik/css/colors.css
index 8443b530f5c..d327c474803 100644
--- a/core/themes/bartik/css/colors.css
+++ b/core/themes/bartik/css/colors.css
@@ -6,14 +6,14 @@ body {
 }
 #page,
 #main-wrapper,
-.region-primary-menu .menu-item a.active,
+.region-primary-menu .menu-item a.is-active,
 .region-primary-menu .menu-item .menu-item--active-trail a {
   background: #ffffff;
 }
-.tabs ul.primary li a.active {
+.tabs ul.primary li a.is-active {
   background-color: #ffffff;
 }
-.tabs ul.primary li.active a {
+.tabs ul.primary li.is-active a {
   background-color: #ffffff;
   border-bottom-color: #ffffff;
 }
@@ -45,7 +45,7 @@ a:active,
 }
 .region-header,
 .region-header a,
-.region-header li a.active,
+.region-header li a.is-active,
 #name-and-slogan,
 .site-branding-block,
 #name-and-slogan a,
diff --git a/core/themes/bartik/css/components/primary-menu.css b/core/themes/bartik/css/components/primary-menu.css
index 44de8e7989f..8b68e85eecc 100644
--- a/core/themes/bartik/css/components/primary-menu.css
+++ b/core/themes/bartik/css/components/primary-menu.css
@@ -44,7 +44,7 @@
   background: #b3b3b3;
   background: rgba(255, 255, 255, 1);
 }
-.region-primary-menu .menu-item a.active {
+.region-primary-menu .menu-item a.is-active {
   border-bottom: none;
 }
 
@@ -182,7 +182,7 @@ body:not(:target) .region-primary-menu .menu-toggle-target-show:target ~ .menu .
     padding: 0.7em 0.8em;
   }
   .featured .region-primary-menu .menu-item a:active,
-  .featured .region-primary-menu .menu-item a.active {
+  .featured .region-primary-menu .menu-item a.is-active {
     background: #f0f0f0;
     background: rgba(240, 240, 240, 1.0);
   }
diff --git a/core/themes/bartik/css/components/site-footer.css b/core/themes/bartik/css/components/site-footer.css
index b0c8814d257..9ed78abe35f 100644
--- a/core/themes/bartik/css/components/site-footer.css
+++ b/core/themes/bartik/css/components/site-footer.css
@@ -57,7 +57,7 @@
   padding-left: 0;
 }
 .site-footer .content a,
-.site-footer .content a.active {
+.site-footer .content a.is-active {
   color: #fcfcfc;
   color: rgba(255, 255, 255, 0.8);
 }
diff --git a/core/themes/bartik/css/components/tabs.css b/core/themes/bartik/css/components/tabs.css
index 4e8051ee61a..31becddb03e 100644
--- a/core/themes/bartik/css/components/tabs.css
+++ b/core/themes/bartik/css/components/tabs.css
@@ -21,7 +21,7 @@ div.tabs {
   margin: 0;
   text-shadow: 0 1px 0 #fff;
 }
-.tabs ul.primary li.active a {
+.tabs ul.primary li.is-active a {
   background-color: #ffffff;
   border: 1px solid #bbb;
 }
@@ -36,7 +36,7 @@ div.tabs {
   .tabs ul.primary li a {
     padding: 5px 10px;
   }
-  .tabs ul.primary li.active a {
+  .tabs ul.primary li.is-active a {
     border-bottom: none;
   }
 }
@@ -67,7 +67,7 @@ div.tabs {
     border-top-left-radius: 6px;
     border-top-right-radius: 6px;
   }
-  .tabs ul.primary li.active a {
+  .tabs ul.primary li.is-active a {
     border-bottom: 1px solid #fff;
   }
 }
@@ -106,7 +106,7 @@ div.tabs {
   padding: 0.25em 0.5em;
   text-decoration: none;
 }
-.tabs ul.secondary li a.active {
+.tabs ul.secondary li a.is-active {
   background: #f2f2f2;
   border-bottom: none;
   border-radius: 5px;
diff --git a/core/themes/classy/templates/dataset/table.html.twig b/core/themes/classy/templates/dataset/table.html.twig
index 518c10cac63..f8e6a6c852d 100644
--- a/core/themes/classy/templates/dataset/table.html.twig
+++ b/core/themes/classy/templates/dataset/table.html.twig
@@ -62,7 +62,7 @@
         {% for cell in header %}
           {%
             set cell_classes = [
-              cell.active_table_sort ? 'active',
+              cell.active_table_sort ? 'is-active',
             ]
           %}
           <{{ cell.tag }}{{ cell.attributes.addClass(cell_classes) }}>
diff --git a/core/themes/seven/css/components/tables.css b/core/themes/seven/css/components/tables.css
index e5f81cf6b59..afc9301bd71 100644
--- a/core/themes/seven/css/components/tables.css
+++ b/core/themes/seven/css/components/tables.css
@@ -77,38 +77,38 @@ th > a:after {
   -webkit-transition: all 0.1s;
   transition: all 0.1s;
 }
-th.active > a {
+th.is-active > a {
   color: #004875;
 }
-th.active img {
+th.is-active img {
   position: absolute;
   right: 0; /* LTR */
   top: 50%;
 }
-[dir="rtl"] th.active img {
+[dir="rtl"] th.is-active img {
   right: auto;
   left: 0;
 }
-th.active > a:after {
+th.is-active > a:after {
   border-bottom-color: #004875;
 }
 th > a:hover,
 th > a:focus,
-th.active > a:hover,
-th.active > a:focus {
+th.is-active > a:hover,
+th.is-active > a:focus {
   color: #008ee6;
   text-decoration: none;
 }
 th > a:hover:after,
 th > a:focus:after,
-th.active > a:hover:after,
-th.active > a:focus:after {
+th.is-active > a:hover:after,
+th.is-active > a:focus:after {
   border-bottom-color: #008ee6;
 }
 td .item-list ul {
   margin: 0;
 }
-td.active {
+td.is-active {
   background: none;
 }
 
diff --git a/core/themes/seven/css/components/tabs.css b/core/themes/seven/css/components/tabs.css
index 78b6d636adc..c7517c4aaca 100644
--- a/core/themes/seven/css/components/tabs.css
+++ b/core/themes/seven/css/components/tabs.css
@@ -81,14 +81,14 @@ li.tabs__tab a {
   margin: 16px 0 0;
   margin: 1rem 0 0;
 }
-.tabs.primary .tabs__tab.active {
+.tabs.primary .tabs__tab.is-active {
   z-index: 15;
   border-color: #a6a6a6;
   border-radius: 4px 0 0 0; /* LTR */
   background-color: #ffffff;
   color: #004f80;
 }
-[dir="rtl"] .tabs.primary .tabs__tab.active {
+[dir="rtl"] .tabs.primary .tabs__tab.is-active {
   border-top-left-radius: 0;
   border-top-right-radius: 4px;
 }
@@ -100,7 +100,7 @@ li.tabs__tab a {
   background-color: #fafaf7;
   text-decoration: underline;
 }
-.tabs.primary .active a:focus {
+.tabs.primary .is-active a:focus {
   background: none;
   text-decoration: underline;
 }
@@ -113,7 +113,7 @@ li.tabs__tab a {
   [dir="rtl"] .tabs.primary a {
     background: url(../../../../misc/icons/0074bd/chevron-left.svg) 1% center no-repeat;
   }
-  .tabs.primary .tabs__tab.active a {
+  .tabs.primary .tabs__tab.is-active a {
     background-image: none;
   }
 }
@@ -157,21 +157,21 @@ li.tabs__tab a {
   padding-bottom:16px;
   padding-bottom: 1rem;
 }
-.is-collapse-enabled .tabs__tab.active {
+.is-collapse-enabled .tabs__tab.is-active {
   position: absolute;
   top: 2px;
   left: 0; /* LTR */
   width: 75%;
   border-bottom: 0;
 }
-[dir="rtl"] .is-collapse-enabled .tabs__tab.active {
+[dir="rtl"] .is-collapse-enabled .tabs__tab.is-active {
   left: auto;
   right: 0;
 }
-.is-collapse-enabled .tabs.primary a.active:before {
+.is-collapse-enabled .tabs.primary a.is-active:before {
   content: none;
 }
-.is-open .tabs__tab.active {
+.is-open .tabs__tab.is-active {
   border-color: #a6a6a6;
   background-color: #ffffff;
   color: #004f80;
@@ -218,9 +218,9 @@ li.tabs__tab a {
 }
 
 /* Override the states above */
-.is-horizontal .tabs__tab.active,
-.is-horizontal .tabs.primary .tabs__tab.active,
-[dir="rtl"] .is-horizontal .tabs.primary .tabs__tab.active {
+.is-horizontal .tabs__tab.is-active,
+.is-horizontal .tabs.primary .tabs__tab.is-active,
+[dir="rtl"] .is-horizontal .tabs.primary .tabs__tab.is-active {
   border-radius: 4px 4px 0 0;
   position: relative;
   width: auto;
@@ -259,12 +259,12 @@ li.tabs__tab a {
 .tabs.secondary .tabs__tab + .tabs__tab {
   border-top: 1px solid #d9d8d4;
 }
-.tabs.secondary .tabs__tab.active {
+.tabs.secondary .tabs__tab.is-active {
   color: #004f80;
   border-left: 2px solid #004f80; /* LTR */
   padding-left: 15px; /* LTR */
 }
-[dir="rtl"] .tabs.secondary .tabs__tab.active {
+[dir="rtl"] .tabs.secondary .tabs__tab.is-active {
   border-left: 1px solid #bfbfbf;
   border-right: 2px solid #004f80;
   padding-right: 15px;
@@ -286,7 +286,7 @@ li.tabs__tab a {
   padding: 7px 13px 5px;
   text-decoration: none;
 }
-.tabs.secondary .active a {
+.tabs.secondary .is-active a {
   color: #004f80;
 }
 .tabs.secondary a:focus {
@@ -319,7 +319,7 @@ li.tabs__tab a {
   border-left-color: transparent;
   padding-right: 0; /* 1 */
 }
-.is-horizontal .tabs.secondary .tabs__tab.active {
+.is-horizontal .tabs.secondary .tabs__tab.is-active {
   border-bottom-color: #004f80;
 }
 .is-horizontal .tabs.secondary .tabs__tab:hover,
diff --git a/core/themes/seven/css/components/views-ui.css b/core/themes/seven/css/components/views-ui.css
index 960d008388b..8c4afda11e7 100644
--- a/core/themes/seven/css/components/views-ui.css
+++ b/core/themes/seven/css/components/views-ui.css
@@ -96,8 +96,8 @@ details.fieldset-no-legend {
 }
 
 .views-displays ul.secondary li a,
-.views-displays ul.secondary li.active a,
-.views-displays ul.secondary li.active a.active {
+.views-displays ul.secondary li.is-active a,
+.views-displays ul.secondary li.is-active a.is-active {
   padding: 2px 7px 3px;
 }
 
@@ -105,8 +105,8 @@ details.fieldset-no-legend {
   color: #0074bd;
 }
 
-.views-displays ul.secondary li.active a,
-.views-displays ul.secondary li.active a.active {
+.views-displays ul.secondary li.is-active a,
+.views-displays ul.secondary li.is-active a.is-active {
   border: 1px solid transparent;
 }
 
diff --git a/core/themes/seven/css/theme/maintenance-page.css b/core/themes/seven/css/theme/maintenance-page.css
index 2d9c30d4f84..678434365c0 100644
--- a/core/themes/seven/css/theme/maintenance-page.css
+++ b/core/themes/seven/css/theme/maintenance-page.css
@@ -66,12 +66,12 @@
   [dir="rtl"] .task-list li {
     padding: 0.5em 3.85em 0.5em 1em;
   }
-  .task-list .active {
+  .task-list .is-active {
     background: #ebeae4;
     position: relative;
     font-weight: normal;
   }
-  .task-list .active:after {
+  .task-list .is-active:after {
     left: 100%; /* LTR */
     border: solid transparent;
     border-color: rgba(235, 234, 228, 0);
@@ -85,7 +85,7 @@
     top: 50%;
     margin-top: -1.32em;
   }
-  [dir="rtl"] .task-list .active:after {
+  [dir="rtl"] .task-list .is-active:after {
     left: auto;
     right: 100%;
     border-left-color: transparent;
diff --git a/core/themes/seven/js/mobile.install.js b/core/themes/seven/js/mobile.install.js
index 7830720c1ac..43196cb9bf3 100644
--- a/core/themes/seven/js/mobile.install.js
+++ b/core/themes/seven/js/mobile.install.js
@@ -4,7 +4,7 @@
 
   function findActiveStep(steps) {
     for (var i = 0; i < steps.length; i++) {
-      if (steps[i].className === 'active') {
+      if (steps[i].className === 'is-active') {
         return i + 1;
       }
     }