KSES: Add `flex` and related long form properties to safe CSS.

Allow `flex`, `flex-grow`, `flex-shrink` and `flex-basis` to be used in inline CSS. As of WordPress 5.3 the block editor is expected to use `flex-basis` inline to set the width in the column block.

Props aduth.
Fixes #47281.
See #37248.



git-svn-id: https://develop.svn.wordpress.org/trunk@45363 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/kses.php

@@ -2063,6 +2063,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 	 * @since 5.0.0 Added support for `background-image`.
 	 * @since 5.1.0 Added support for `text-transform`.
 	 * @since 5.2.0 Added support for `background-position` and `grid-template-columns`
+	 * @since 5.3.0 Added support for `flex`, `flex-grow`, `flex-shrink`, and `flex-basis`.
 	 *
 	 * @param string[] $attr Array of allowed CSS attributes.
 	 */
@@ -2133,6 +2134,11 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'padding-left',
 			'padding-top',
 
+			'flex',
+			'flex-grow',
+			'flex-shrink',
+			'flex-basis',
+
 			'clear',
 			'cursor',
 			'direction',

tests/phpunit/tests/kses.php

@@ -830,6 +830,11 @@ EOF;
 				'css'      => 'background: green url("foo.jpg") no-repeat fixed center',
 				'expected' => 'background: green url("foo.jpg") no-repeat fixed center',
 			),
+			// `flex` and related attributes introduced in 5.3.
+			array(
+				'css'      => 'flex: 0 1 auto;flex-basis: 75%;flex-shrink: 0;flex-grow: 1',
+				'expected' => 'flex: 0 1 auto;flex-basis: 75%;flex-shrink: 0;flex-grow: 1',
+			),
 		);
 	}