core/includes/common.inc                           |   10 +++----
 .../Drupal/Core/Utility/LinkGeneratorInterface.php |   11 +++----
 .../Drupal/system/Controller/SystemController.php  |   31 +++++++++++++++-----
 3 files changed, 34 insertions(+), 18 deletions(-)

diff --git a/core/includes/common.inc b/core/includes/common.inc
index 0a77f0d..24d52a0 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -1214,11 +1214,11 @@ function drupal_http_header_attributes(array $attributes = array()) {
  *     internal to the site, $options['language'] is used to determine whether
  *     the link is "active", or pointing to the current page (the language as
  *     well as the path must match). This element is also used by url().
- *   - 'set_active_class' (default FALSE): Whether l() should compare the $path,
- *     language and query options to the current URL to determine whether the
- *     link is "active". If so, an "active" class will be applied to the link.
- *     It is important to use this sparingly since it is usually unnecessary and
- *     requires extra processing.
+ *   - 'set_active_class': Whether l() should compare the $path, language and
+ *     query options to the current URL to determine whether the link is
+ *     "active". Defaults to FALSE. If TRUE, an "active" class will be applied
+ *     to the link. It is important to use this sparingly since it is usually
+ *     unnecessary and requires extra processing.
  *     For anonymous users, the "active" class will be calculated on the server,
  *     because most sites serve each anonymous user the same cached page anyway.
  *     For authenticated users, the "active" class will be calculated on the
diff --git a/core/lib/Drupal/Core/Utility/LinkGeneratorInterface.php b/core/lib/Drupal/Core/Utility/LinkGeneratorInterface.php
index b832873..02e31af 100644
--- a/core/lib/Drupal/Core/Utility/LinkGeneratorInterface.php
+++ b/core/lib/Drupal/Core/Utility/LinkGeneratorInterface.php
@@ -55,11 +55,12 @@
    *     internal to the site, $options['language'] is used to determine whether
    *     the link is "active", or pointing to the current page (the language as
    *     well as the path must match).
-   *   - 'set_active_class' (default FALSE): Whether this method should compare
-   *     the $route_name, $parameters, language and query options to the current
-   *     URL to determine whether the link is "active". If so, an "active" class
-   *     will be applied to the link. It is important to use this sparingly
-   *     since it is usually unnecessary and requires extra processing.
+   *   - 'set_active_class': Whether this method should compare the $route_name,
+   *     $parameters, language and query options to the current URL to determine
+   *     whether the link is "active". Defaults to FALSE. If TRUE, an "active"
+   *     class will be applied to the link. It is important to use this
+   *     sparingly since it is usually unnecessary and requires extra
+   *     processing.
    *
    * @return string
    *   An HTML string containing a link to the given route and parameters.
diff --git a/core/modules/system/lib/Drupal/system/Controller/SystemController.php b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
index d6a1de9..e16c838 100644
--- a/core/modules/system/lib/Drupal/system/Controller/SystemController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
@@ -373,7 +373,7 @@ public static function setLinkActiveClass(array $element, array $context) {
         $pos_match = $pos_current_path;
         $type_match = 'path';
       }
-      else if ($context['front'] && $pos_front !== FALSE) {
+      elseif ($context['front'] && $pos_front !== FALSE) {
         $pos_match = $pos_front;
         $type_match = 'front';
       }
@@ -399,7 +399,7 @@ public static function setLinkActiveClass(array $element, array $context) {
       // Parse it into a DOMDocument so we can reliably read and modify
       // attributes.
       $dom = new \DOMDocument();
-      $dom->loadHTML('<!DOCTYPE html><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head><body>' . $tag . '</body></html>');
+      @$dom->loadHTML('<!DOCTYPE html><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head><body>' . $tag . '</body></html>');
       $node = $dom->getElementsByTagName('body')->item(0)->firstChild;
 
       // The language of an active link is equal to the current language.
@@ -411,9 +411,16 @@ public static function setLinkActiveClass(array $element, array $context) {
       }
       // The query parameters of an active link are equal to the current
       // parameters.
-      if ($is_active && $context['query']) {
-        if ($node->hasAttribute('data-drupal-link-query') && $node->getAttribute('data-drupal-link-query') !== Json::encode($context['query'])) {
-          $is_active = FALSE;
+      if ($is_active) {
+        if ($context['query']) {
+          if (!$node->hasAttribute('data-drupal-link-query') || $node->getAttribute('data-drupal-link-query') !== Json::encode($context['query'])) {
+            $is_active = FALSE;
+          }
+        }
+        else {
+          if ($node->hasAttribute('data-drupal-link-query')) {
+            $is_active = FALSE;
+          }
         }
       }
 
@@ -430,11 +437,19 @@ public static function setLinkActiveClass(array $element, array $context) {
         $body_dom_node = $dom->getElementsByTagName('body')->item(0);
         $updated_tag = substr($dom->saveHTML($body_dom_node), 6, -7);
 
+        // If saveHTML() added a closing tag (it does so for <a>), remove it.
+        if (substr_count($updated_tag, '<') > 1) {
+          $updated_tag = substr($updated_tag, 0, strrpos($updated_tag, '<'));
+        }
         $element['#markup'] = str_replace($tag, $updated_tag, $element['#markup']);
-      }
 
-      // Ensure we only search the remaining HTML.
-      $offset = $pos_tag_end - strlen($tag) + strlen($updated_tag);
+        // Ensure we only search the remaining HTML.
+        $offset = $pos_tag_end - strlen($tag) + strlen($updated_tag);
+      }
+      else {
+        // Ensure we only search the remaining HTML.
+        $offset = $pos_tag_end + 1;
+      }
     }
 
     return $element;