diff --git a/core/modules/filter/src/Plugin/Filter/FilterHtml.php b/core/modules/filter/src/Plugin/Filter/FilterHtml.php
index 6682f51..38d798a 100644
--- a/core/modules/filter/src/Plugin/Filter/FilterHtml.php
+++ b/core/modules/filter/src/Plugin/Filter/FilterHtml.php
@@ -131,37 +131,10 @@ public function filterAttributes($text) {
       }
       krsort($allowed_attributes['prefix']);
 
-      // Find all nodes that have any attributes and filter the attributes by
-      // name and value.
-      foreach ($xpath->query('//' . $allowed_tag . '[@*]') as $node) {
-        $modified_attributes = [];
-        foreach($node->attributes as $name => $attribute) {
-          // Remove attributes not in the whitelist.
-          $allowed_value = $this->findAllowedValue($allowed_attributes, $name);
-          if (empty($allowed_value)) {
-            $modified_attributes[$name] = FALSE;
-          }
-          elseif ($allowed_value !== TRUE) {
-            // Check the attribute values whitelist.
-            $attribute_values = preg_split('/\s+/', $attribute->value, -1, PREG_SPLIT_NO_EMPTY);
-            $modified_attributes[$name] = [];
-            foreach ($attribute_values as $value) {
-              if ($this->findAllowedValue($allowed_value, $value)) {
-                $modified_attributes[$name][] = $value;
-              }
-            }
-          }
-        }
-        // If the $allowed_value was TRUE for an attribute name, it does not
-        // appear in this array so the value on the DOM node is left unchanged.
-        foreach ($modified_attributes as $name => $values) {
-          if ($values) {
-            $node->setAttribute($name, implode(' ', $values));
-          }
-          else {
-            $node->removeAttribute($name);
-          }
-        }
+      // Find all matching elements that have any attributes and filter the
+      // attributes by name and value.
+      foreach ($xpath->query('//' . $allowed_tag . '[@*]') as $element) {
+        $this->filterElementAttributes($element, $allowed_attributes);
       }
     }
 
@@ -177,6 +150,45 @@ public function filterAttributes($text) {
   }
 
   /**
+   * Filter attributes on an element by name and value according to a whitelist.
+   *
+   * @param \DOMElement $element
+   *   The element to be processed.
+   * @param array $allowed_attributes
+   *   The attributes whitelist as an array of names and values.
+   */
+  protected function filterElementAttributes(\DOMElement $element, array $allowed_attributes) {
+    $modified_attributes = [];
+    foreach($element->attributes as $name => $attribute) {
+      // Remove attributes not in the whitelist.
+      $allowed_value = $this->findAllowedValue($allowed_attributes, $name);
+      if (empty($allowed_value)) {
+        $modified_attributes[$name] = FALSE;
+      }
+      elseif ($allowed_value !== TRUE) {
+        // Check the attribute values whitelist.
+        $attribute_values = preg_split('/\s+/', $attribute->value, -1, PREG_SPLIT_NO_EMPTY);
+        $modified_attributes[$name] = [];
+        foreach ($attribute_values as $value) {
+          if ($this->findAllowedValue($allowed_value, $value)) {
+            $modified_attributes[$name][] = $value;
+          }
+        }
+      }
+    }
+    // If the $allowed_value was TRUE for an attribute name, it does not
+    // appear in this array so the value on the DOM element is left unchanged.
+    foreach ($modified_attributes as $name => $values) {
+      if ($values) {
+        $element->setAttribute($name, implode(' ', $values));
+      }
+      else {
+        $element->removeAttribute($name);
+      }
+    }
+  }
+
+  /**
    * Helper function to handle prefix matching.
    *
    * @param array $allowed
@@ -256,6 +268,8 @@ public function getHTMLRestrictions() {
       }
       $tag = $node->tagName;
       if ($node->hasAttributes()) {
+        // Mark the tag as allowed, assigning TRUE for each attribute name if
+        // all values are allowed, or an array of specific allowed values.
         $restrictions['allowed'][$tag] = [];
         // Iterate over any attributes, and mark them as allowed.
         foreach ($node->attributes as $name => $attribute) {