diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index ec97db2..d93dd0f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -601,8 +601,8 @@ function template_preprocess_datetime_wrapper(&$variables) {
  *       included, it must be an array of one or more class names.
  *     If the 'href' element is supplied, the entire link array is passed to
  *     l() as its $options parameter.
- *   - attributes: A keyed array of attributes for the <ul> containing the list
- *     of links.
+ *   - attributes: A keyed array of attributes for the <ul> containing the
+ *     list of links.
  *   - set_active_class: (optional) Whether each link should compare the
  *     route_name + route_parameters or href (path), language and query options
  *     to the current URL, to determine whether the link is "active". If so, an
@@ -631,6 +631,15 @@ function template_preprocess_datetime_wrapper(&$variables) {
  *     http://juicystudio.com/article/screen-readers-display-none.php and
  *     http://www.w3.org/TR/WCAG-TECHS/H42.html for more information.
  *
+ * Unfortunately links templates duplicate the "active" class handling of l()
+ * and LinkGenerator::generate() because it needs to be able to set the "active"
+ * class not on the links themselves (<a> tags), but on the list items (<li>
+ * tags) that contain the links. This is necessary for CSS to be able to style
+ * 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.is-active")')
+ *
  * @see \Drupal\Core\Utility\LinkGenerator
  * @see \Drupal\Core\Utility\LinkGenerator::generate()
  * @see system_page_attachments()
@@ -642,7 +651,7 @@ function template_preprocess_links(&$variables) {
   if (!empty($links)) {
     // Prepend the heading to the list, if any.
     if (!empty($heading)) {
-      // Convert a string heading into an array, using a H2 tag by default.
+      // Convert a string heading into an array, using a <h2> tag by default.
       if (is_string($heading)) {
         $heading = array('text' => $heading);
       }
@@ -844,28 +853,28 @@ function template_preprocess_image(&$variables) {
  *   - colgroups: An array of column groups. Each element of the array can be
  *     either:
  *     - An array of columns, each of which is an associative array of HTML
- *       attributes applied to the COL element.
- *     - An array of attributes applied to the COLGROUP element, which must
- *       include a "data" attribute. To add attributes to COL elements, set the
+ *       attributes applied to the <col> element.
+ *     - An array of attributes applied to the <colgroup> element, which must
+ *       include a "data" attribute. To add attributes to <col> elements, set the
  *       "data" attribute with an array of columns, each of which is an
  *       associative array of HTML attributes.
  *     Here's an example for $colgroup:
  *     @code
  *     $colgroup = array(
- *       // COLGROUP with one COL element.
+ *       // <colgroup> with one <col> element.
  *       array(
  *         array(
- *           'class' => array('funky'), // Attribute for the COL element.
+ *           'class' => array('funky'), // Attribute for the <col> element.
  *         ),
  *       ),
- *       // Colgroup with attributes and inner COL elements.
+ *       // <colgroup> with attributes and inner <col> elements.
  *       array(
  *         'data' => array(
  *           array(
- *             'class' => array('funky'), // Attribute for the COL element.
+ *             'class' => array('funky'), // Attribute for the <col> element.
  *           ),
  *         ),
- *         'class' => array('jazzy'), // Attribute for the COLGROUP element.
+ *         'class' => array('jazzy'), // Attribute for the <colgroup> element.
  *       ),
  *     );
  *     @endcode
@@ -1106,6 +1115,23 @@ function template_preprocess_item_list(&$variables) {
 }
 
 /**
+ * Returns HTML for an indentation <div>; used for drag and drop tables.
+ *
+ * @param $variables
+ *   An associative array containing:
+ *   - size: Optional. The number of indentations to create.
+ *
+ * @ingroup themeable
+ */
+function theme_indentation($variables) {
+  $output = '';
+  for ($n = 0; $n < $variables['size']; $n++) {
+    $output .= '<div class="js-indentation indentation">&nbsp;</div>';
+  }
+  return $output;
+}
+
+/**
  * Prepares variables for container templates.
  *
  * Default template: container.html.twig.
@@ -1143,7 +1169,7 @@ function template_preprocess_container(&$variables) {
  *   An associative array containing:
  *   - items: An associative array of maintenance tasks.
  *     It's the caller's responsibility to ensure this array's items contain no
- *     dangerous HTML such as SCRIPT tags.
+ *     dangerous HTML such as <script> tags.
  *   - active: The key for the currently active maintenance task.
  */
 function template_preprocess_maintenance_task_list(&$variables) {
@@ -1253,7 +1279,7 @@ function template_preprocess_html(&$variables) {
 
   $variables['html_attributes'] = new Attribute();
 
-  // HTML element attributes.
+  // <html> element attributes.
   $language_interface = \Drupal::languageManager()->getCurrentLanguage();
   $variables['html_attributes']['lang'] = $language_interface->getId();
   $variables['html_attributes']['dir'] = $language_interface->getDirection();
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 5001554..0f9e53b 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -1036,7 +1036,7 @@
       // called with a context object that includes top-level text nodes.
       // However, to give developers full control of the HTML appearing in the
       // page, and to enable Ajax content to be inserted in places where DIV
-      // elements are not allowed (e.g., within TABLE, TR, and SPAN parents),
+      // elements are not allowed (e.g., within <table>, <tr>, and <span> parents),
       // we check if the new content satisfies the requirement of a single
       // top-level element, and only use the container DIV created above when
       // it doesn't. For more information, please see
diff --git a/core/misc/tableselect.js b/core/misc/tableselect.js
index 053af05..243e000 100644
--- a/core/misc/tableselect.js
+++ b/core/misc/tableselect.js
@@ -101,7 +101,7 @@
       // range. Also make sure that we are actually checking checkboxes
       // over a range and that a checkbox has been checked or unchecked before.
       if (e.shiftKey && lastChecked && lastChecked !== e.target) {
-        // We use the checkbox's parent TR to do our range searching.
+        // We use the checkbox's parent <tr> to do our range searching.
         Drupal.tableSelectRange($(e.target).closest('tr')[0], $(lastChecked).closest('tr')[0], e.target.checked);
       }
 
diff --git a/core/modules/system/templates/fieldset.html.twig b/core/modules/system/templates/fieldset.html.twig
index b67ec85..efda588 100644
--- a/core/modules/system/templates/fieldset.html.twig
+++ b/core/modules/system/templates/fieldset.html.twig
@@ -4,12 +4,12 @@
  * Default theme implementation for a fieldset element and its children.
  *
  * Available variables:
- * - attributes: HTML attributes for the fieldset element.
+ * - attributes: HTML attributes for the <fieldset> element.
  * - errors: (optional) Any errors for this fieldset element, may not be set.
  * - required: Boolean indicating whether the fieldeset element is required.
  * - legend: The legend element containing the following properties:
  *   - title: Title of the fieldset, intended for use as the text of the legend.
- *   - attributes: HTML attributes to apply to the legend.
+ *   - attributes: HTML attributes to apply to the <legend> element.
  * - description: The description element containing the following properties:
  *   - content: The description content of the fieldset.
  *   - attributes: HTML attributes to apply to the description container.
@@ -38,7 +38,7 @@
       required ? 'form-required',
     ]
   %}
-  {#  Always wrap fieldset legends in a SPAN for CSS positioning. #}
+  {#  Always wrap fieldset legends in a <span> for CSS positioning. #}
   <legend{{ legend.attributes }}>
     <span{{ legend_span.attributes.addClass(legend_span_classes) }}>{{ legend.title }}</span>
   </legend>
diff --git a/core/modules/system/templates/region.html.twig b/core/modules/system/templates/region.html.twig
index e009455..219e14b 100644
--- a/core/modules/system/templates/region.html.twig
+++ b/core/modules/system/templates/region.html.twig
@@ -5,7 +5,7 @@
  *
  * Available variables:
  * - content: The content for this region, typically blocks.
- * - attributes: HTML attributes for the region div.
+ * - attributes: HTML attributes for the region <div>.
  * - region: The name of the region variable as defined in the theme's
  *   .info.yml file.
  *
diff --git a/core/modules/system/templates/select.html.twig b/core/modules/system/templates/select.html.twig
index fb7226e..0b5c3d2 100644
--- a/core/modules/system/templates/select.html.twig
+++ b/core/modules/system/templates/select.html.twig
@@ -4,8 +4,8 @@
  * Default theme implementation for a select element.
  *
  * Available variables:
- * - attributes: HTML attributes for the select tag.
- * - options: The option element children.
+ * - attributes: HTML attributes for the <select> tag.
+ * - options: The <option> element children.
  *
  * @see template_preprocess_select()
  *
diff --git a/core/modules/system/templates/table.html.twig b/core/modules/system/templates/table.html.twig
index f6b32e4..e13edbd 100644
--- a/core/modules/system/templates/table.html.twig
+++ b/core/modules/system/templates/table.html.twig
@@ -12,7 +12,7 @@
  *     https://www.drupal.org/node/893530 and
  *     http://api.drupal.org/api/drupal/includes!theme.inc/function/theme_table/7#comment-5109.
  * - header: Table header cells. Each cell contains the following properties:
- *   - tag: The HTML tag name to use; either TH or TD.
+ *   - tag: The HTML tag name to use; either 'th' or 'td' for a <th> or <td> cell respectively.
  *   - attributes: HTML attributes to apply to the tag.
  *   - content: A localized string for the title of the column.
  *   - field: Field name (required for column sorting).
@@ -24,7 +24,7 @@
  *   - no_striping: A flag indicating that the row should receive no
  *     'even / odd' styling. Defaults to FALSE.
  *   - cells: Table cells of the row. Each cell contains the following keys:
- *     - tag: The HTML tag name to use; either TH or TD.
+ *     - tag: The HTML tag name to use; either 'th' or 'td' for a <th> or <td> cell respectively.
  *     - attributes: Any HTML attributes, such as "colspan", to apply to the
  *       table cell.
  *     - content: The string to display in the table cell.
diff --git a/core/modules/system/templates/textarea.html.twig b/core/modules/system/templates/textarea.html.twig
index 6a1a46e..cd987d2 100644
--- a/core/modules/system/templates/textarea.html.twig
+++ b/core/modules/system/templates/textarea.html.twig
@@ -5,7 +5,7 @@
  *
  * Available variables
  * - wrapper_attributes: A list of HTML attributes for the wrapper element.
- * - attributes: A list of HTML attributes for the textarea element.
+ * - attributes: A list of HTML attributes for the <textarea> element.
  * - resizable: An indicator for whether the textarea is resizable.
  * - required: An indicator for whether the textarea is required.
  * - value: The textarea content.
diff --git a/core/themes/classy/templates/dataset/table.html.twig b/core/themes/classy/templates/dataset/table.html.twig
index 70801d6..aa47e72 100644
--- a/core/themes/classy/templates/dataset/table.html.twig
+++ b/core/themes/classy/templates/dataset/table.html.twig
@@ -12,7 +12,7 @@
  *     https://www.drupal.org/node/893530 and
  *     http://api.drupal.org/api/drupal/includes!theme.inc/function/theme_table/7#comment-5109.
  * - header: Table header cells. Each cell contains the following properties:
- *   - tag: The HTML tag name to use; either TH or TD.
+ *   - tag: The HTML tag name to use; either 'th' or 'td' for a <th> or <td> cell respectively.
  *   - attributes: HTML attributes to apply to the tag.
  *   - content: A localized string for the title of the column.
  *   - field: Field name (required for column sorting).
@@ -24,7 +24,7 @@
  *   - no_striping: A flag indicating that the row should receive no
  *     'even / odd' styling. Defaults to FALSE.
  *   - cells: Table cells of the row. Each cell contains the following keys:
- *     - tag: The HTML tag name to use; either TH or TD.
+ *     - tag: The HTML tag name to use; either 'th' or 'td' for a <th> or <td> cell respectively.
  *     - attributes: Any HTML attributes, such as "colspan", to apply to the
  *       table cell.
  *     - content: The string to display in the table cell.
diff --git a/core/themes/classy/templates/form/fieldset.html.twig b/core/themes/classy/templates/form/fieldset.html.twig
index 4bd7d0a..0d089ed 100644
--- a/core/themes/classy/templates/form/fieldset.html.twig
+++ b/core/themes/classy/templates/form/fieldset.html.twig
@@ -36,7 +36,7 @@
       required ? 'form-required',
     ]
   %}
-  {#  Always wrap fieldset legends in a SPAN for CSS positioning. #}
+  {#  Always wrap fieldset legends in a <span> for CSS positioning. #}
   <legend{{ legend.attributes }}>
     <span{{ legend_span.attributes.addClass(legend_span_classes) }}>{{ legend.title }}</span>
   </legend>
diff --git a/core/themes/classy/templates/form/select.html.twig b/core/themes/classy/templates/form/select.html.twig
index 3150bdc..8af4261 100644
--- a/core/themes/classy/templates/form/select.html.twig
+++ b/core/themes/classy/templates/form/select.html.twig
@@ -4,8 +4,8 @@
  * Theme override for a select element.
  *
  * Available variables:
- * - attributes: HTML attributes for the select tag.
- * - options: The option element children.
+ * - attributes: HTML attributes for the <select> tag.
+ * - options: The <option> element children.
  *
  * @see template_preprocess_select()
  */
diff --git a/core/themes/classy/templates/form/textarea.html.twig b/core/themes/classy/templates/form/textarea.html.twig
index 24e647b..99e1bde 100644
--- a/core/themes/classy/templates/form/textarea.html.twig
+++ b/core/themes/classy/templates/form/textarea.html.twig
@@ -5,7 +5,7 @@
  *
  * Available variables
  * - wrapper_attributes: A list of HTML attributes for the wrapper element.
- * - attributes: A list of HTML attributes for the textarea element.
+ * - attributes: A list of HTML attributes for the <textarea> element.
  * - resizable: An indicator for whether the textarea is resizable.
  * - required: An indicator for whether the textarea is required.
  * - value: The textarea content.
diff --git a/core/themes/classy/templates/layout/region.html.twig b/core/themes/classy/templates/layout/region.html.twig
index cff654a..95e71ce 100644
--- a/core/themes/classy/templates/layout/region.html.twig
+++ b/core/themes/classy/templates/layout/region.html.twig
@@ -5,7 +5,7 @@
  *
  * Available variables:
  * - content: The content for this region, typically blocks.
- * - attributes: HTML attributes for the region div.
+ * - attributes: HTML attributes for the region <div>.
  * - region: The name of the region variable as defined in the theme's
  *   .info.yml file.
  *
diff --git a/core/themes/seven/css/components/tables.css b/core/themes/seven/css/components/tables.css
index 3bdf7cc..913e21a 100644
--- a/core/themes/seven/css/components/tables.css
+++ b/core/themes/seven/css/components/tables.css
@@ -116,7 +116,7 @@ td.is-active {
   background: none;
 }
 
-/* Force browsers to calculate the width of a 'select all' TH element. */
+/* Force browsers to calculate the width of a 'select all' <th> element. */
 th.select-all {
   width: 1px;
 }