KSES: Add opacity to the list of safe CSS properties.

Original PR from Gutenberg repository:
* [WordPress/gutenberg#59891 #59891 Update overlay step function in cover block]

Reference: [https://developer.mozilla.org/en-US/docs/Web/CSS/opacity MDN Web Docs: opacity].

Props sunil25393, wildworks, poena, Mamaduka, presstoke.
Fixes #61536.

git-svn-id: https://develop.svn.wordpress.org/trunk@58709 602fd350-edb4-49c9-b593-d223f7449a82

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',

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',
+			),
 		);
 	}