diff --git a/src/wp-includes/kses.php b/src/wp-includes/kses.php
index 0ef5803ebc7b4..cd30d845f83af 100644
--- a/src/wp-includes/kses.php
+++ b/src/wp-includes/kses.php
@@ -2526,6 +2526,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'list-style-type',
 			'object-fit',
 			'object-position',
+			'opacity',
 			'overflow',
 			'vertical-align',
 			'writing-mode',
diff --git a/tests/phpunit/tests/kses.php b/tests/phpunit/tests/kses.php
index ea65a89092c07..b5cca81047172 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',
 			),
+			// `opacity` introduced in 6.7.
+			array(
+				'css'      => 'opacity: 10',
+				'expected' => 'opacity: 10',
+			),
 		);
 	}