diff -u b/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
--- b/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -333,7 +333,7 @@
    * {@inheritdoc}
    */
   public function count() {
-    return mb_strlen($this->__toString());
+    return count($this->__toString());
   }
 
   /**
only in patch2:
unchanged:
--- a/core/lib/Drupal/Component/Render/FormattableMarkup.php
+++ b/core/lib/Drupal/Component/Render/FormattableMarkup.php
@@ -58,7 +58,7 @@
  * @see \Drupal\Core\StringTranslation\PluralTranslatableMarkup
  * @see \Drupal\Component\Render\FormattableMarkup::placeholderFormat()
  */
-class FormattableMarkup implements MarkupInterface, \Countable {
+class FormattableMarkup implements MarkupInterface {
 
   /**
    * The string containing placeholders.
only in patch2:
unchanged:
--- a/core/lib/Drupal/Component/Render/HtmlEscapedText.php
+++ b/core/lib/Drupal/Component/Render/HtmlEscapedText.php
@@ -12,7 +12,7 @@
  *
  * @ingroup sanitization
  */
-class HtmlEscapedText implements MarkupInterface, \Countable {
+class HtmlEscapedText implements MarkupInterface {
 
   /**
    * The string to escape.
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Field/FieldFilteredMarkup.php
+++ b/core/lib/Drupal/Core/Field/FieldFilteredMarkup.php
@@ -19,7 +19,7 @@
  *
  * @see \Drupal\Core\Render\Markup
  */
-final class FieldFilteredMarkup implements MarkupInterface, \Countable {
+final class FieldFilteredMarkup implements MarkupInterface {
   use MarkupTrait;
 
   /**
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/GeneratedLink.php
+++ b/core/lib/Drupal/Core/GeneratedLink.php
@@ -11,7 +11,7 @@
  * Note: not to be confused with \Drupal\Core\Link, which is for passing around
  *   ungenerated links (typically link text + route name + route parameters).
  */
-class GeneratedLink extends BubbleableMetadata implements MarkupInterface, \Countable {
+class GeneratedLink extends BubbleableMetadata implements MarkupInterface {
 
   /**
    * HTML tag to use when building the link.
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Render/Markup.php
+++ b/core/lib/Drupal/Core/Render/Markup.php
@@ -19,7 +19,7 @@
  * @see \Drupal\Core\Template\TwigExtension::escapeFilter
  * @see \Twig\Markup
  */
-final class Markup implements MarkupInterface, \Countable {
+final class Markup implements MarkupInterface {
   use MarkupTrait;
 
 }
only in patch2:
unchanged:
--- a/core/modules/big_pipe/src/Render/BigPipeMarkup.php
+++ b/core/modules/big_pipe/src/Render/BigPipeMarkup.php
@@ -18,7 +18,7 @@
  *
  * @see \Drupal\Core\Render\Markup
  */
-final class BigPipeMarkup implements MarkupInterface, \Countable {
+final class BigPipeMarkup implements MarkupInterface {
   use MarkupTrait;
 
 }
only in patch2:
unchanged:
--- a/core/modules/filter/src/Render/FilteredMarkup.php
+++ b/core/modules/filter/src/Render/FilteredMarkup.php
@@ -19,7 +19,7 @@
  *
  * @see \Drupal\Core\Render\Markup
  */
-final class FilteredMarkup implements MarkupInterface, \Countable {
+final class FilteredMarkup implements MarkupInterface {
   use MarkupTrait;
 
 }
only in patch2:
unchanged:
--- a/core/modules/migrate/src/Audit/AuditResult.php
+++ b/core/modules/migrate/src/Audit/AuditResult.php
@@ -8,7 +8,7 @@
 /**
  * Encapsulates the result of a migration audit.
  */
-class AuditResult implements MarkupInterface, \Countable {
+class AuditResult implements MarkupInterface {
 
   /**
    * The audited migration.
only in patch2:
unchanged:
--- a/core/modules/views/src/Render/ViewsRenderPipelineMarkup.php
+++ b/core/modules/views/src/Render/ViewsRenderPipelineMarkup.php
@@ -18,7 +18,7 @@
  *
  * @see \Drupal\Core\Render\Markup
  */
-final class ViewsRenderPipelineMarkup implements MarkupInterface, \Countable {
+final class ViewsRenderPipelineMarkup implements MarkupInterface {
   use MarkupTrait;
 
 }