diff --git a/core/lib/Drupal/Component/Utility/SafeMarkup.php b/core/lib/Drupal/Component/Utility/SafeMarkup.php
index 6257f98..7da361f 100644
--- a/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -285,12 +285,12 @@ public static function placeholder($text) {
   /**
    * Replace all occurrences of the search string with the replacement string.
    *
-   * Functions identically to str_replace, but marks the returned output as safe
-   * if all the inputs and the subject have also been marked as safe.
+   * Functions identically to str_replace(), but marks the returned output as
+   * safe if all the inputs and the subject have also been marked as safe.
    *
    * @param string|array $search
-   *   The value being searched for, an array may be used to designate multiple
-   *   values.
+   *   The value being searched for. An array may be used to designate multiple
+   *   values to search for.
    * @param string|array $replace
    *   The replacement value that replaces found search values. An array may be
    *   used to designate multiple replacements.
@@ -309,6 +309,8 @@ public static function replace($search, $replace, $subject) {
       }
     }
     else {
+      // If any replacement is unsafe, then the output is also unsafe, so just
+      // return the output.
       foreach ($replace as $replacement) {
         if (!SafeMarkup::isSafe($replacement)) {
           return $output;
@@ -319,11 +321,11 @@ public static function replace($search, $replace, $subject) {
     // If we have reached this point, then all replacements were safe, and
     // therefore if the subject was also safe, then the entire output is also
     // safe, and should be marked as such.
-    if (SafeMarkup::isSafe($subject)) {
-      return SafeMarkup::set($output);
+    if (!SafeMarkup::isSafe($subject)) {
+      return $output;
     }
     else {
-      return $output;
+      return SafeMarkup::set($output);
     }
   }
 
diff --git a/core/modules/contextual/src/Element/ContextualLinksPlaceholder.php b/core/modules/contextual/src/Element/ContextualLinksPlaceholder.php
index e148a1b..a0c2739 100644
--- a/core/modules/contextual/src/Element/ContextualLinksPlaceholder.php
+++ b/core/modules/contextual/src/Element/ContextualLinksPlaceholder.php
@@ -48,10 +48,9 @@ public function getInfo() {
    * @see _contextual_links_to_id()
    */
   public static function preRenderPlaceholder(array $element) {
-    // Because the only arguments to this markup will be instance of
+    // This markup is safe because the arguments will always be instances of
     // \Drupal\Core\Template\AttributeString, which is passed through
-    // \Drupal\Component\Utility\SafeMarkup::checkPlain() before being output
-    // this markup is safe, and is marked as such.
+    // \Drupal\Component\Utility\SafeMarkup::checkPlain() before being output.
     $element['#markup'] = SafeMarkup::set('<div' . new Attribute(array('data-contextual-id' => $element['#id'])) . '></div>');
     return $element;
   }
diff --git a/core/modules/filter/src/Element/ProcessedText.php b/core/modules/filter/src/Element/ProcessedText.php
index 4851a22..51c8b83 100644
--- a/core/modules/filter/src/Element/ProcessedText.php
+++ b/core/modules/filter/src/Element/ProcessedText.php
@@ -119,8 +119,8 @@ public static function preRenderText($element) {
       }
     }
 
-    // Filtering and sanitizing has been done in
-    // \Drupal\filter\Plugin\FilterInterface. Store its content in #markup,
+    // Filtering and sanitizing have been done in
+    // \Drupal\filter\Plugin\FilterInterface. Store the content in #markup,
     // set the updated bubbleable rendering metadata, and set the text format's
     // cache tag.
     $element['#markup'] = SafeMarkup::set($text);