diff --git a/CHANGES b/CHANGES
index 49782fb4686..1b06ad12506 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4,6 +4,9 @@ Release 7.2.2 (in development)
 Bugs fixed
 ----------
 
+* Fixed membership testing (``in``) for the :py:class:`str` interface
+  of the asset classes (``_CascadingStyleSheet`` and ``_JavaScript``),
+  which several extensions relied upon.
 
 Release 7.2.1 (released Aug 17, 2023)
 =====================================
diff --git a/sphinx/builders/html/_assets.py b/sphinx/builders/html/_assets.py
index 2d9247007a1..a72c5000bbc 100644
--- a/sphinx/builders/html/_assets.py
+++ b/sphinx/builders/html/_assets.py
@@ -36,6 +36,10 @@ def __str__(self):
                 f'{attr})')
 
     def __eq__(self, other):
+        if isinstance(other, str):
+            warnings.warn('The str interface for _CascadingStyleSheet objects is deprecated. '
+                          'Use css.filename instead.', RemovedInSphinx90Warning, stacklevel=2)
+            return self.filename == other
         if not isinstance(other, _CascadingStyleSheet):
             return NotImplemented
         return (self.filename == other.filename
@@ -88,6 +92,10 @@ def __str__(self):
                 f'{attr})')
 
     def __eq__(self, other):
+        if isinstance(other, str):
+            warnings.warn('The str interface for _JavaScript objects is deprecated. '
+                          'Use js.filename instead.', RemovedInSphinx90Warning, stacklevel=2)
+            return self.filename == other
         if not isinstance(other, _JavaScript):
             return NotImplemented
         return (self.filename == other.filename