reverted:
--- b/core/includes/tablesort.inc
+++ a/core/includes/tablesort.inc
@@ -50,7 +50,7 @@
       // 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 -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1214,7 +1214,7 @@
     $variables['tasks'][$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;
     }
@@ -1539,7 +1539,7 @@
     $variables['tasks'][$k]['item'] = $item;
     $variables['tasks'][$k]['attributes'] = new Attribute();
     if ($active == $k) {
-      $variables['tasks'][$k]['attributes']->addClass('is-active');
+      $variables['tasks'][$k]['attributes']->addClass('active');
       $variables['tasks'][$k]['status'] = t('active');
       $done = FALSE;
     }
diff -u b/core/modules/toolbar/js/toolbar.menu.js b/core/modules/toolbar/js/toolbar.menu.js
--- b/core/modules/toolbar/js/toolbar.menu.js
+++ b/core/modules/toolbar/js/toolbar.menu.js
@@ -128,7 +128,7 @@
         activeItem = location.pathname;
       }
       if (activeItem) {
-        var $activeItem = $menu.find('a[href="' + activeItem + '"]').addClass('is-active');
+        var $activeItem = $menu.find('a[href="' + activeItem + '"]').addClass('active');
         var $activeTrail = $activeItem.parentsUntil('.root', 'li').addClass('active-trail');
         toggleList($activeTrail, true);
       }
@@ -148,7 +148,7 @@
         activeItem = location.pathname;
       }
       if (activeItem) {
-        var $activeItem = $menu.find('a[href="' + activeItem + '"]').addClass('active');
+        var $activeItem = $menu.find('a[href="' + activeItem + '"]').addClass('is-active');
         var $activeTrail = $activeItem.parentsUntil('.root', 'li').addClass('active-trail');
         toggleList($activeTrail, true);
       }
diff -u b/core/modules/views_ui/css/views_ui.admin.theme.css b/core/modules/views_ui/css/views_ui.admin.theme.css
--- b/core/modules/views_ui/css/views_ui.admin.theme.css
+++ b/core/modules/views_ui/css/views_ui.admin.theme.css
@@ -442,7 +442,7 @@
 }
 
 .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;
@@ -450,7 +450,7 @@
 }
 
 .views-displays .tabs.secondary li,
-.views-displays .tabs.secondary li.is-active {
+.views-displays .tabs.secondary li.active {
   background: transparent;
   margin-bottom: 5px;
   border: 0;
@@ -479,7 +479,7 @@
 /**
  * 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;
@@ -487,23 +487,23 @@
 /**
  * Display a red border if the display doesn't validate.
  */
-.views-displays .tabs.secondary li.is-active a.is-active.error,
+.views-displays .tabs.secondary li.active a.active.error,
 .views-displays .tabs.secondary a.error {
   border: 2px solid #ed541d;
   padding: 1px 6px;
 }
 
 .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;
 }
 }
 
 .views-displays .tabs.secondary li a:hover,
-.views-displays .tabs.secondary li.is-active a,
-.views-displays .tabs.secondary li.is-active a.is-active {
+.views-displays .tabs.secondary li.active a,
+.views-displays .tabs.secondary li.active a.active {
   background-color: #555;
   color: #fff;
 }
diff -u b/core/themes/bartik/css/style.css b/core/themes/bartik/css/style.css
--- b/core/themes/bartik/css/style.css
+++ b/core/themes/bartik/css/style.css
@@ -2018,7 +2018,7 @@
     padding: 0.7em 0.8em;
   }
   .featured .region-primary-menu .menu li a:active,
-  .featured .region-primary-menu .menu li a.is-active {
+  .featured .region-primary-menu .menu li a.active {
     background: #f0f0f0;
     background: rgba(240, 240, 240, 1.0);
   }
@@ -2119,7 +2119,7 @@
     padding: 0.7em 0.8em;
   }
   .featured .region-primary-menu .menu li a:active,
-  .featured .region-primary-menu .menu li a.active {
+  .featured .region-primary-menu .menu li a.is-active {
     background: #f0f0f0;
     background: rgba(240, 240, 240, 1.0);
   }
reverted:
--- b/core/themes/seven/css/components/tables.css
+++ a/core/themes/seven/css/components/tables.css
@@ -66,38 +66,38 @@
   -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;
 }