class-amp-style-sanitizer.php.diff

diff --git a/includes/sanitizers/class-amp-style-sanitizer.php b/includes/sanitizers/class-amp-style-sanitizer.php
index 988c4942..87d0c470 100644
--- a/includes/sanitizers/class-amp-style-sanitizer.php
+++ b/includes/sanitizers/class-amp-style-sanitizer.php
@@ -667,6 +667,15 @@ class AMP_Style_Sanitizer extends AMP_Base_Sanitizer {
 
 				$this->used_attributes[ $attribute_name ] = ( 0 !== $this->xpath->query( $expression )->length );
 			}
+
+			// Attributes for amp-accordion, see <https://amp.dev/documentation/components/amp-accordion/#styling>.
+			if ( 'expanded' === $attribute_name ) {
+				if ( ! $this->has_used_tag_names( [ 'amp-accordion' ] ) ) {
+					return false;
+				}
+				continue;
+			}
+
 			if ( ! $this->used_attributes[ $attribute_name ] ) {
 				return false;
 			}