diff --git a/src/wp-includes/kses.php b/src/wp-includes/kses.php
index 0ef5803ebc7b4..3efb951a7b643 100644
--- a/src/wp-includes/kses.php
+++ b/src/wp-includes/kses.php
@@ -2539,6 +2539,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'box-shadow',
 			'aspect-ratio',
 			'container-type',
+			'opacity',
 
 			// Custom CSS properties.
 			'--*',
diff --git a/tests/phpunit/tests/kses.php b/tests/phpunit/tests/kses.php
index ea65a89092c07..ec8a3bc5335a9 100644
--- a/tests/phpunit/tests/kses.php
+++ b/tests/phpunit/tests/kses.php
@@ -1353,6 +1353,11 @@ public function data_safecss_filter_attr() {
 				'css'      => 'background-repeat: no-repeat',
 				'expected' => 'background-repeat: no-repeat',
 			),
+			// `background-repeat` introduced in 6.7.
+			array(
+				'css'      => 'opacity: 10',
+				'expected' => 'opacity: 10',
+			),
 		);
 	}