diff --git a/includes/sanitizers/class-amp-core-theme-sanitizer.php b/includes/sanitizers/class-amp-core-theme-sanitizer.php
index f24804dace6..85b9a3b0c79 100644
--- a/includes/sanitizers/class-amp-core-theme-sanitizer.php
+++ b/includes/sanitizers/class-amp-core-theme-sanitizer.php
@@ -198,8 +198,7 @@ public static function get_supported_themes() {
 	public static function get_acceptable_errors( $template ) {
 		if ( isset( self::$theme_features[ $template ] ) ) {
 			return [
-				'removed_unused_css_rules' => true,
-				'illegal_css_at_rule'      => [
+				'illegal_css_at_rule' => [
 					[
 						'at_rule' => 'viewport',
 					],
diff --git a/includes/validation/class-amp-validated-url-post-type.php b/includes/validation/class-amp-validated-url-post-type.php
index 8bf9bc83614..3ed2d4beb2e 100644
--- a/includes/validation/class-amp-validated-url-post-type.php
+++ b/includes/validation/class-amp-validated-url-post-type.php
@@ -1622,11 +1622,7 @@ public static function handle_validation_error_status_update() {
 			'amp_taxonomy_terms_updated' => $updated_count,
 		];
 
-		/*
-		 * Re-check the post after the validation status change. This is particularly important for validation errors like
-		 * 'removed_unused_css_rules' since whether it is accepted will determine whether other validation errors are triggered
-		 * such as in this case 'excessive_css'.
-		 */
+		// Re-check the post after the validation status change.
 		if ( $updated_count > 0 ) {
 			$validation_results = self::recheck_post( $post->ID );
 			// @todo For WP_Error case, see <https://github.com/ampproject/amp-wp/issues/1166>.
diff --git a/includes/validation/class-amp-validation-error-taxonomy.php b/includes/validation/class-amp-validation-error-taxonomy.php
index a18d8cf6f03..39e77a32891 100644
--- a/includes/validation/class-amp-validation-error-taxonomy.php
+++ b/includes/validation/class-amp-validation-error-taxonomy.php
@@ -2325,8 +2325,6 @@ public static function get_error_title_from_code( $error_code ) {
 				return __( 'Illegal CSS at-rule', 'amp' );
 			case 'disallowed_file_extension':
 				return __( 'Disallowed file extension', 'amp' );
-			case 'removed_unused_css_rules':
-				return __( 'Remove unused CSS rules', 'amp' ); // @todo This is obsolete.
 			default:
 				return __( 'Unknown Error', 'amp' );
 		}