KSES: Add writing-mode to the list of safe CSS properties.

Original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/54581 #54581 Gutenberg Plugin: Add hook to allow `writing-mode` as a safe CSS property]

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

Follow-up to [56605].

Props wildworks, mukesh27, poena, andrewserong.
Fixes #59387.

git-svn-id: https://develop.svn.wordpress.org/trunk@56617 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/kses.php

@@ -2301,6 +2301,7 @@ function kses_init() {
  *              and `z-index` CSS properties.
  * @since 6.3.0 Extended support for `filter` to accept a URL and added support for repeat().
  *              Added support for `box-shadow`.
+ * @since 6.4.0 Added support for `writing-mode`.
  *
  * @param string $css        A string of CSS rules.
  * @param string $deprecated Not used.
@@ -2461,6 +2462,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'object-position',
 			'overflow',
 			'vertical-align',
+			'writing-mode',
 
 			'position',
 			'top',

tests/phpunit/tests/kses.php

@@ -1341,6 +1341,11 @@ EOF;
 				'css'      => 'grid-template-columns: repeat(4, unsupported(0, 1fr)',
 				'expected' => '',
 			),
+			// `writing-mode` introduced in 6.4.
+			array(
+				'css'      => 'writing-mode: vertical-rl',
+				'expected' => 'writing-mode: vertical-rl',
+			),
 		);
 	}