diff --git a/Alert.php b/Alert.php
index e5755c1..18f3960 100644
--- a/Alert.php
+++ b/Alert.php
@@ -135,9 +135,7 @@ class Alert extends Widget
      */
     protected function initOptions()
     {
-        Html::addCssClass($this->options, 'alert');
-        Html::addCssClass($this->options, 'fade');
-        Html::addCssClass($this->options, 'in');
+        Html::addCssClass($this->options, ['alert', 'fade', 'in']);
 
         if ($this->closeButton !== false) {
             $this->closeButton = array_merge([
diff --git a/Button.php b/Button.php
index eedcc46..985279a 100644
--- a/Button.php
+++ b/Button.php
@@ -46,7 +46,7 @@ class Button extends Widget
     {
         parent::init();
         $this->clientOptions = false;
-        Html::addCssClass($this->options, 'btn');
+        Html::addCssClass($this->options, ['widget' => 'btn']);
     }
 
     /**
diff --git a/ButtonDropdown.php b/ButtonDropdown.php
index 0d10b6f..1a379d7 100644
--- a/ButtonDropdown.php
+++ b/ButtonDropdown.php
@@ -75,7 +75,7 @@ class ButtonDropdown extends Widget
     public function run()
     {
         // @todo use [[options]] instead of [[containerOptions]] and introduce [[buttonOptions]] before 2.1 release
-        Html::addCssClass($this->containerOptions, 'btn-group');
+        Html::addCssClass($this->containerOptions, ['widget' => 'btn-group']);
         $options = $this->containerOptions;
         $tag = ArrayHelper::remove($options, 'tag', 'div');
 
@@ -94,7 +94,7 @@ class ButtonDropdown extends Widget
      */
     protected function renderButton()
     {
-        Html::addCssClass($this->options, 'btn');
+        Html::addCssClass($this->options, ['widget' => 'btn']);
         $label = $this->label;
         if ($this->encodeLabel) {
             $label = Html::encode($label);
@@ -102,7 +102,7 @@ class ButtonDropdown extends Widget
         if ($this->split) {
             $options = $this->options;
             $this->options['data-toggle'] = 'dropdown';
-            Html::addCssClass($this->options, 'dropdown-toggle');
+            Html::addCssClass($this->options, ['toggle' => 'dropdown-toggle']);
             unset($this->options['id']);
             $splitButton = Button::widget([
                 'label' => '<span class="caret"></span>',
@@ -116,7 +116,7 @@ class ButtonDropdown extends Widget
             if (!isset($options['href'])) {
                 $options['href'] = '#';
             }
-            Html::addCssClass($options, 'dropdown-toggle');
+            Html::addCssClass($options, ['toggle' => 'dropdown-toggle']);
             $options['data-toggle'] = 'dropdown';
             $splitButton = '';
         }
diff --git a/ButtonGroup.php b/ButtonGroup.php
index 4bf47ef..a0c3ec2 100644
--- a/ButtonGroup.php
+++ b/ButtonGroup.php
@@ -65,7 +65,7 @@ class ButtonGroup extends Widget
     public function init()
     {
         parent::init();
-        Html::addCssClass($this->options, 'btn-group');
+        Html::addCssClass($this->options, ['widget' => 'btn-group']);
     }
 
     /**
diff --git a/Carousel.php b/Carousel.php
index f212619..d932a66 100644
--- a/Carousel.php
+++ b/Carousel.php
@@ -72,7 +72,7 @@ class Carousel extends Widget
     public function init()
     {
         parent::init();
-        Html::addCssClass($this->options, 'carousel');
+        Html::addCssClass($this->options, ['widget' => 'carousel']);
     }
 
     /**
@@ -149,7 +149,7 @@ class Carousel extends Widget
             throw new InvalidConfigException('The "content" option is required.');
         }
 
-        Html::addCssClass($options, 'item');
+        Html::addCssClass($options, ['widget' => 'item']);
         if ($index === 0) {
             Html::addCssClass($options, 'active');
         }
diff --git a/Collapse.php b/Collapse.php
index 0b33cd5..b2de61f 100644
--- a/Collapse.php
+++ b/Collapse.php
@@ -78,7 +78,7 @@ class Collapse extends Widget
     public function init()
     {
         parent::init();
-        Html::addCssClass($this->options, 'panel-group');
+        Html::addCssClass($this->options, ['widget' => 'panel-group']);
     }
 
     /**
@@ -109,7 +109,7 @@ class Collapse extends Widget
             }
             $header = $item['label'];
             $options = ArrayHelper::getValue($item, 'options', []);
-            Html::addCssClass($options, 'panel panel-default');
+            Html::addCssClass($options, ['panel' => 'panel', 'widget' => 'panel-default']);
             $items[] = Html::tag('div', $this->renderItem($header, $item, ++$index), $options);
         }
 
@@ -130,7 +130,7 @@ class Collapse extends Widget
             $id = $this->options['id'] . '-collapse' . $index;
             $options = ArrayHelper::getValue($item, 'contentOptions', []);
             $options['id'] = $id;
-            Html::addCssClass($options, 'panel-collapse collapse');
+            Html::addCssClass($options, ['widget' => 'panel-collapse', 'collapse' => 'collapse']);
 
             $encodeLabel = isset($item['encode']) ? $item['encode'] : $this->encodeLabels;
             if ($encodeLabel) {
diff --git a/Dropdown.php b/Dropdown.php
index 1c28f01..b68ac11 100644
--- a/Dropdown.php
+++ b/Dropdown.php
@@ -77,7 +77,7 @@ class Dropdown extends Widget
             unset($this->submenuOptions['id']);
         }
         parent::init();
-        Html::addCssClass($this->options, 'dropdown-menu');
+        Html::addCssClass($this->options, ['widget' => 'dropdown-menu']);
     }
 
     /**
@@ -120,7 +120,7 @@ class Dropdown extends Widget
             if (empty($item['items'])) {
                 if ($url === null) {
                     $content = $label;
-                    Html::addCssClass($itemOptions, 'dropdown-header');
+                    Html::addCssClass($itemOptions, ['widget' => 'dropdown-header']);
                 } else {
                     $content = Html::a($label, $url, $linkOptions);
                 }
@@ -131,7 +131,7 @@ class Dropdown extends Widget
                 }
                 $content = Html::a($label, $url === null ? '#' : $url, $linkOptions)
                     . $this->renderItems($item['items'], $submenuOptions);
-                Html::addCssClass($itemOptions, 'dropdown-submenu');
+                Html::addCssClass($itemOptions, ['widget' => 'dropdown-submenu']);
             }
 
             $lines[] = Html::tag('li', $content, $itemOptions);
diff --git a/Modal.php b/Modal.php
index c138f30..6555135 100644
--- a/Modal.php
+++ b/Modal.php
@@ -136,7 +136,7 @@ class Modal extends Widget
             $this->header = $button . "\n" . $this->header;
         }
         if ($this->header !== null) {
-            Html::addCssClass($this->headerOptions, 'modal-header');
+            Html::addCssClass($this->headerOptions, ['widget' => 'modal-header']);
             return Html::tag('div', "\n" . $this->header . "\n", $this->headerOptions);
         } else {
             return null;
@@ -168,7 +168,7 @@ class Modal extends Widget
     protected function renderFooter()
     {
         if ($this->footer !== null) {
-            Html::addCssClass($this->footerOptions, 'modal-footer');
+            Html::addCssClass($this->footerOptions, ['widget' => 'modal-footer']);
             return Html::tag('div', "\n" . $this->footer . "\n", $this->footerOptions);
         } else {
             return null;
@@ -224,7 +224,7 @@ class Modal extends Widget
             'role' => 'dialog',
             'tabindex' => -1,
         ], $this->options);
-        Html::addCssClass($this->options, 'modal');
+        Html::addCssClass($this->options, ['widget' => 'modal']);
 
         if ($this->clientOptions !== false) {
             $this->clientOptions = array_merge(['show' => false], $this->clientOptions);
diff --git a/Nav.php b/Nav.php
index 5be7994..d59e9b0 100644
--- a/Nav.php
+++ b/Nav.php
@@ -121,7 +121,7 @@ class Nav extends Widget
         if ($this->dropDownCaret === null) {
             $this->dropDownCaret = Html::tag('b', '', ['class' => 'caret']);
         }
-        Html::addCssClass($this->options, 'nav');
+        Html::addCssClass($this->options, ['widget' => 'nav']);
     }
 
     /**
@@ -178,8 +178,8 @@ class Nav extends Widget
 
         if ($items !== null) {
             $linkOptions['data-toggle'] = 'dropdown';
-            Html::addCssClass($options, 'dropdown');
-            Html::addCssClass($linkOptions, 'dropdown-toggle');
+            Html::addCssClass($options, ['widget' => 'dropdown']);
+            Html::addCssClass($linkOptions, ['widget' => 'dropdown-toggle']);
             if ($this->dropDownCaret !== '') {
                 $label .= ' ' . $this->dropDownCaret;
             }
diff --git a/NavBar.php b/NavBar.php
index aa6db21..f67f32b 100644
--- a/NavBar.php
+++ b/NavBar.php
@@ -93,11 +93,11 @@ class NavBar extends Widget
     {
         parent::init();
         $this->clientOptions = false;
-        Html::addCssClass($this->options, 'navbar');
-        if ($this->options['class'] === 'navbar') {
-            Html::addCssClass($this->options, 'navbar-default');
+        if (empty($this->options['class'])) {
+            Html::addCssClass($this->options, ['navbar', 'navbar-default']);
+        } else {
+            Html::addCssClass($this->options, ['widget' => 'navbar']);
         }
-        Html::addCssClass($this->brandOptions, 'navbar-brand');
         if (empty($this->options['role'])) {
             $this->options['role'] = 'navigation';
         }
@@ -116,12 +116,11 @@ class NavBar extends Widget
         }
         echo $this->renderToggleButton();
         if ($this->brandLabel !== false) {
-            Html::addCssClass($this->brandOptions, 'navbar-brand');
+            Html::addCssClass($this->brandOptions, ['widget' => 'navbar-brand']);
             echo Html::a($this->brandLabel, $this->brandUrl === false ? Yii::$app->homeUrl : $this->brandUrl, $this->brandOptions);
         }
         echo Html::endTag('div');
-        Html::addCssClass($this->containerOptions, 'collapse');
-        Html::addCssClass($this->containerOptions, 'navbar-collapse');
+        Html::addCssClass($this->containerOptions, ['collapse' => 'collapse', 'widget' => 'navbar-collapse']);
         $options = $this->containerOptions;
         $tag = ArrayHelper::remove($options, 'tag', 'div');
         echo Html::beginTag($tag, $options);
diff --git a/Progress.php b/Progress.php
index d80bd6b..14c7122 100644
--- a/Progress.php
+++ b/Progress.php
@@ -96,7 +96,7 @@ class Progress extends Widget
     public function init()
     {
         parent::init();
-        Html::addCssClass($this->options, 'progress');
+        Html::addCssClass($this->options, ['widget' => 'progress']);
     }
 
     /**
@@ -152,7 +152,7 @@ class Progress extends Widget
             'style' => "width:{$percent}%",
         ];
         $options = array_merge($defaultOptions, $options);
-        Html::addCssClass($options, 'progress-bar');
+        Html::addCssClass($options, ['widget' => 'progress-bar']);
 
         $out = Html::beginTag('div', $options);
         $out .= $label;
diff --git a/Tabs.php b/Tabs.php
index ff5293a..f60724f 100644
--- a/Tabs.php
+++ b/Tabs.php
@@ -122,7 +122,7 @@ class Tabs extends Widget
     public function init()
     {
         parent::init();
-        Html::addCssClass($this->options, 'nav ' . $this->navType);
+        Html::addCssClass($this->options, ['widget' => 'nav', $this->navType]);
     }
 
     /**
@@ -162,13 +162,13 @@ class Tabs extends Widget
 
             if (isset($item['items'])) {
                 $label .= ' <b class="caret"></b>';
-                Html::addCssClass($headerOptions, 'dropdown');
+                Html::addCssClass($headerOptions, ['widget' => 'dropdown']);
 
                 if ($this->renderDropdown($n, $item['items'], $panes)) {
                     Html::addCssClass($headerOptions, 'active');
                 }
 
-                Html::addCssClass($linkOptions, 'dropdown-toggle');
+                Html::addCssClass($linkOptions, ['widget' => 'dropdown-toggle']);
                 $linkOptions['data-toggle'] = 'dropdown';
                 $header = Html::a($label, "#", $linkOptions) . "\n"
                     . Dropdown::widget(['items' => $item['items'], 'clientOptions' => false, 'view' => $this->getView()]);
@@ -176,7 +176,7 @@ class Tabs extends Widget
                 $options = array_merge($this->itemOptions, ArrayHelper::getValue($item, 'options', []));
                 $options['id'] = ArrayHelper::getValue($options, 'id', $this->options['id'] . '-tab' . $n);
 
-                Html::addCssClass($options, 'tab-pane');
+                Html::addCssClass($options, ['widget' => 'tab-pane']);
                 if (ArrayHelper::remove($item, 'active')) {
                     Html::addCssClass($options, 'active');
                     Html::addCssClass($headerOptions, 'active');
@@ -241,7 +241,7 @@ class Tabs extends Widget
 
             $content = ArrayHelper::remove($item, 'content');
             $options = ArrayHelper::remove($item, 'contentOptions', []);
-            Html::addCssClass($options, 'tab-pane');
+            Html::addCssClass($options, ['widget' => 'tab-pane']);
             if (ArrayHelper::remove($item, 'active')) {
                 Html::addCssClass($options, 'active');
                 Html::addCssClass($item['options'], 'active');
diff --git a/docs/guide/usage-widgets.md b/docs/guide/usage-widgets.md
index 004a949..6b63e72 100644
--- a/docs/guide/usage-widgets.md
+++ b/docs/guide/usage-widgets.md
@@ -17,3 +17,40 @@ framework features. All widgets belong to `\yii\bootstrap` namespace:
 - [[yii\bootstrap\NavBar|NavBar]]
 - [[yii\bootstrap\Progress|Progress]]
 - [[yii\bootstrap\Tabs|Tabs]]
+
+
+## Customize widget CSS classes <span id="widgets-customize-css-classes"></span>
+
+Widgets allow quick composition of the HTML for particular Bootstrap components, which requires usage of the
+Bootstrap CSS classes. Those classes are added automatically by the widgets. However usually widget provides
+fields or properties, which allow customization of the CSS styles for particular HTML tags.
+
+For example: you may use [[yii\bootstrap\Button::options]] to customize appearance of the rendered button.
+Class 'btn' will be added automatically, so you don't need to worry about it. All you need is specify particular
+button class:
+
+```php
+echo Button::widget([
+    'label' => 'Action',
+    'options' => ['class' => 'btn-primary'], // produces class "btn btn-primary"
+]);
+```
+
+However, sometimes you may need to replace standard class, added by the widget, with alternative one.
+For example: widget [[yii\bootstrap\ButtonGroup]] uses 'btn-group' class for the container div, but you may
+need to use 'btn-group-vertical', which allows vertical-style layout for the buttons. Using plain 'class' option
+simply adds 'btn-group-vertical' to 'btn-group', which produces incorrect result.
+In order to actually replace class added by widget, you need to specify 'class' option as array, containing
+needed class name under the 'widget' key:
+
+```php
+echo ButtonGroup::widget([
+    'options' => [
+        'class' => ['widget' => 'btn-group-vertical'] // replace 'btn-group' with 'btn-group-vertical'
+    ],
+    'buttons' => [
+        ['label' => 'A'],
+        ['label' => 'B'],
+    ]
+]);
+```