diff --git src/wp-includes/kses.php src/wp-includes/kses.php
index 2b96e84a47..d73c5e8ff1 100644
--- src/wp-includes/kses.php
+++ 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',