diff --git a/src/purifier/sanitation.js b/src/purifier/sanitation.js
index 6ecb2fd4d969..9c65b2614858 100644
--- a/src/purifier/sanitation.js
+++ b/src/purifier/sanitation.js
@@ -235,6 +235,7 @@ export const ALLOWLISTED_ATTRS = [
   'max-font-size',
   'on',
   'option',
+  'expanded',
   'placeholder',
   // Attributes related to amp-form.
   'submitting',
diff --git a/test/unit/test-purifier.js b/test/unit/test-purifier.js
index ceadbdca3778..7b88bc1c0c1e 100644
--- a/test/unit/test-purifier.js
+++ b/test/unit/test-purifier.js
@@ -193,6 +193,12 @@ describes.sandboxed('DOMPurify-based', {}, (env) => {
       expect(rewriteAttributeValueSpy.callCount).to.be.equal(1);
     });
 
+    it('should output "expanded" attribute', () => {
+      expect(purify('<section expanded>Header</section>')).to.equal(
+        '<section expanded="">Header</section>'
+      );
+    });
+
     it('should default target to _top with href', () => {
       // Can't use string equality since DOMPurify will reorder attributes.
       const actual = serialize(