@@ -2526,6 +2526,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'list-style-type',
 			'object-fit',
 			'object-position',
+			'opacity',
 			'overflow',
 			'vertical-align',
 			'writing-mode',
 
@@ -1353,6 +1353,11 @@ public function data_safecss_filter_attr() {
 				'css'      => 'background-repeat: no-repeat',
 				'expected' => 'background-repeat: no-repeat',
 			),
+			// `opacity` introduced in 6.7.
+			array(
+				'css'      => 'opacity: 10',
+				'expected' => 'opacity: 10',
+			),
 		);
 	}